THA loading arising from increased femoral anteversion and offset may lead to critical cement stresses

Understanding the role of head size and neck length in micromotion generation at the taper junction in total hip arthroplasty

Modular hip implants allow intra-operative adjustments for patient-specific customization and targeted replacement of damaged elements without full implant extraction. However, challenges arise from relative micromotions between components, potentially leading to implant failure due to cytotoxic metal debris. In this study magnitude and directions of micromotions at the taper junction were estimated, aiming to understand the effect of variations in head size and neck length. Starting from a reference configuration adhering to the 12/14 taper standard, six additional implant configurations were generated by varying the head size and/or neck length. A musculoskeletal multibody model of a prothesized lower limb was developed to estimate hip contact force and location during a normal walking task. Following the implant assembly, the multibody-derived loads were imposed as boundary conditions in a finite element analysis to compute the taper junction micromotions as the relative slip between the contacting surfaces. Results highlighted the L-size head as the most critical configuration, indicating a 2.81 μm relative slip at the mid-stance phase. The proposed approach enables the investigation of geometric variations in implants under accurate load conditions, providing valuable insights for designing less risky prostheses and informing clinical decision-making processes.

Arthroplasty for femoral neck fractures is at risk for under restoration of lateral femoral offset

PURPOSE

The aim of the study was to determine the restoration of hip biomechanics through lateral offset, leg length, and acetabular component position when comparing non-arthroplasty surgeons (NAS) to elective arthroplasty surgeons (EAS).

METHODS

131 patients, with a femoral neck fracture treated with a THA by 7 EAS and 20 NAS, were retrospectively reviewed. 2 blinded observers measured leg-length discrepancy, femoral offset, and acetabular component position. Multivariate logistic regression models examined the association between the surgeon groups and restoration of lateral femoral, acetabular offset, leg length discrepancy, acetabular anteversion, acetabular position, and component size, while adjusting for surgical approach and spinal pathology.

RESULTS

NAS under-restored 4.8 mm of lateral femoral offset (43.9 ± 8.7 mm) after THA when compared to the uninjured side (48.7 ± 7.1 mm, p = 0.044). NAS were at risk for under-restoring lateral femoral offset when compared to EAS (p = 0.040). There was no association between lateral acetabular offset, leg length, acetabular position, or component size and surgeon type.

CONCLUSIONS

Lateral femoral offset is at risk for under-restoration after THA for femoral neck fractures, when performed by surgeons that do not regularly perform elective THA. This indicates that lateral femoral offset is an under-appreciated contributor to hip instability when performing THA for a femoral neck fracture. Lateral femoral offset deserves as much attention and awareness as acetabular component position since a secondary analysis of our data reveal that preoperative templating and intraoperative imaging did not prevent under-restoration.

Biomechanical analysis and clinical observation of 3D-printed acetabular prosthesis for the acetabular reconstruction of total hip arthroplasty in Crowe III hip dysplasia

Objective: This study aimed to evaluate the biomechanical effectiveness of 3D-printed integrated acetabular prosthesis (IAP) and modular acetabular prosthesis (MAP) in reconstructing the acetabulum for patients with Crowe III developmental dysplasia of the hip (DDH). The results of this study can provide a theoretical foundation for the treatment of Crowe III DDH in total hip arthroplasty (THA). Methods: Finite element (FE) analysis models were created to reconstruct Crowe III DDH acetabular defects using IAP and MAP. The contact stress and relative micromotion between the acetabular prosthesis and the host bone were analyzed by gradually loading in three increments (210 N, 2100 N, and 4200 N). In addition, five patients with Crowe III DDH who underwent IAP acetabular reconstruction were observed. Results: At the same load, the peak values of IAP contact stress and relative micromotion were lower than those of MAP acetabular reconstruction. Under jogging load, the MAP metal augment's peak stress exceeded porous tantalum yield strength, and the risk of prosthesis fracture was higher. The peak stress in the bone interface in contact with the MAP during walking and jogging was higher than that in the cancellous bone, while that of IAP was higher than that of the cancellous bone only under jogging load, so the risk of MAP cancellous bone failure was greater. Under jogging load, the relative micromotion of the MAP reconstruction acetabular implant was 45.2 μm, which was not conducive to bone growth, while under three different loads, the relative micromotion of the IAP acetabular implant was 1.5-11.2 μm, all <40 μm, which was beneficial to bone growth. Five patients with IAP acetabular reconstruction were followed up for 11.8 ± 3.4 months, and the Harris score of the last follow-up was 85.4 ± 5.5. The imaging results showed good stability of all prostheses with no adverse conditions observed. Conclusion: Compared with acetabular reconstruction with MAP, IAP has a lower risk of loosening and fracture, as well as a better long-term stability. The application of IAP is an ideal acetabular reconstruction method for Crowe III DDH.

A Modified Modular Stem in Primary Total Hip Arthroplasty for Developmental Dysplasia of the Hip: Average 11-year Follow-Up in Cases With Previously Reported 3-year Clinical Results

BACKGROUND

Modular stems require careful follow-up, especially after any design modification. This study investigated the mid- to long-term outcomes of total hip arthroplasty in patients with developmental dysplasia of the hip using the S-ROM-A stem, a modified S-ROM stem for Asians.

METHODS

We previously reported short-term outcomes for all 220 dysplastic hips that underwent primary total hip arthroplasty with the S-ROM-A stem. Here, we followed the clinical and radiological outcomes of 201 of the 220 hips (91%) for a mean 11.4 years postoperatively. We also performed multivariate analysis to determine whether large anteversion angle adjustment was associated with increased osteolysis.

RESULTS

The cumulative survival rate of the stem at 168 months postoperatively was 97.4%. Two hips underwent revision surgery, including 1 due to neck trunnionosis. Characteristic distal stem fracture occurred in 3 hips (1.5%). Most partial radiolucent lines observed around the sleeve early postoperatively disappeared by 7 years, postoperatively. Mild osteolysis occurred relatively frequently (20%), but a multivariate model adjusted for polyethylene type showed no significant association between the occurrence of osteolysis and anteversion adjustment of the modular stem (stem anteversion decreased ≤-20°, P = .829; stem anteversion increased ≥+20°, P = .619).

CONCLUSION

Partial radiolucent lines early postoperatively do not affect long-term outcomes. The clinical benefits of actively adjusting the stem anteversion angle outweigh the mechanical risks. Mild osteolysis, stem fracture, and trunnionosis were relatively frequent complications. Analysis with longer follow-up and more cases are necessary to clearly determine if these complications are associated with the design modification.

Feature extraction and gait classification in hip replacement patients on the basis of kinematic waveform data

Study aim: To find out, without relying on gait-specific assumptions or prior knowledge, which parameters are most important for the description of asymmetrical gait in patients after total hip arthroplasty (THA).

Material and methods: The gait of 22 patients after THA was recorded using an optical motion capture system. The waveform data of the marker positions, velocities, and accelerations, as well as joint and segment angles, were used as initial features. The random forest (RF) and minimum-redundancy maximum-relevance (mRMR) algorithms were chosen for feature selection. The results were compared with those obtained from the use of different dimensionality reduction methods.

Results: Hip movement in the sagittal plane, knee kinematics in the frontal and sagittal planes, marker position data of the anterior and posterior superior iliac spine, and acceleration data for markers placed at the proximal end of the fibula are highly important for classification (accuracy: 91.09%). With feature selection, better results were obtained compared to dimensionality reduction.

Conclusion: The proposed approaches can be used to identify and individually address abnormal gait patterns during the rehabilitation process via waveform data. The results indicate that position and acceleration data also provide significant information for this task.

Analysis of hip joint loading during walking with different shoe types using instrumented total hip prostheses

Hip joint loads need careful consideration during postoperative physiotherapy after joint replacement. One factor influencing joint loads is the choice of footwear, but it remains unclear which footwear is favorable. The objective of the present study was to investigate the influence of footwear on hip joint loads in vivo. Instrumented hip endoprostheses were used for in vivo load measurements. The parameters resultant contact force (F_res), bending moment (M_bend) and torsional moment (M_tors) were evaluated during treadmill walking at 4 km/h with different shoe types. In general, footwear tended to increase hip joint loading, with the barefoot shoe having the least influence. F_res and M_bend were significantly increased during heel strike for all shoe types in comparison to barefoot walking, with everyday shoe (34.6%; p = 0.028 and 47%; p = 0.028, respectively) and men’s shoe (33.2%; p = 0.043 and 41.1%; p = 0.043, respectively) resulting in the highest changes. M_tors at AbsMax was increased by all shoes except for the barefoot shoe, with the highest changes for men’s shoe (+ 17.6%, p = 0.043) and the shoe with stiffened sole (+ 17.5%, p = 0.08). Shoes, especially those with stiff soles or elaborate cuishing and guiding elements, increase hip joint loads during walking. The influence on peak loads is higher for M_tors than for F_res and M_bend. For patients in which a reduction of hip joints loads is desired, e.g. during physiotherapy after recent surgery or to alleviate symptoms of osteoarthritis, low profile shoes with a flexible sole may be preferred over shoes with a stiff sole or elaborate cushioning elements.

Gender-specific difference of in-vivo kinematics in patients with unilateral total hip arthroplasty

PURPOSE

Asymmetric gait patterns are known to persist in patients with unilateral total hip arthroplasty (THA). Little is known about the effect of gender on gait asymmetry. This study aimed to determine whether gait asymmetry differs between genders in unilateral THA patients and can be explained by component positioning.

METHODS

23 female and 10 male unilateral THA patients were included. Asymmetric hip kinematics during gait and component positioning were quantified using a combined computed tomography-based modelling and dual fluoroscopic imaging system. Multiple regression was performed to examine the unique and mediated contribution of gender to observed gait asymmetry.

RESULTS

Female and male patients differed most significantly in frontal plane gait asymmetry (p = 0.001) and bilateral difference of vertical femoral offset (p = 0.048). Compared with the native hip, the implanted hip exhibited significantly increased adduction (p = 0.007) in females but significantly increased abduction (p = 0.001) in males. The stem head of the implanted hip was more superiorly positioned compared to the native femoral head in female but more inferiorly positioned in male. Gender accounted for 41.5% variance of frontal plane gait asymmetry and was partially mediated by the bilateral difference of vertical femoral offset.

CONCLUSIONS

Female unilateral THA patients exhibited significantly increased adduction in the implanted hip compared to the native hip, which is partially attributable to a more superiorly positioned femoral stem centre of rotation. The understanding of gender-specific differences of kinematic patterns may benefit female patients through targeted preoperative planning and postoperative rehabilitation.

Three dimensional bone mineral density changes in the femur over 1 year in primary total hip arthroplasty patients

BACKGROUND

The aim of the study was to compare the bone mineral density changes between unmatched patients undergoing total hip arthroplasty receiving uncemented and cemented type of implants. Previous studies have used DEXA or a two dimensional analysis to estimate the bone quality following total joint replacement, whereas this study presents the changes in three dimensions.

METHODS

Fifty subjects both male and females receiving both cemented and uncemented type of implant were recruited. Two CT scans were taken of each subject, the first at 24 h post surgery and the second one 1 year after surgery. The scans were calibrated using a phantom converting the Hounsfield units to bone mineral density values in g/cm³. The two scans were registered together using anatomical landmarks and resliced to compare the two femurs in the identical frame of reference. The bone density gain and loss was calculated by comparing density values between the two sets of scans.

FINDINGS

The results showed that most of the bone loss was located around the Lesser Trochanter and some bone density gain at the distal tip of the implant. The three dimensional density changes occur differently between individuals and the study showed no correlation of bone loss with age.

INTERPRETATION

The bone loss occurred mostly at the proximal femur, which is in agreement with previously presented studies. By carrying out three dimensional analysis on the bone gain and loss on the femur, it is possible to identify the patients that are showing high degree of bone loss.

Hip resurfacing generates a more physiological gait than total hip replacement: A case-control study

BACKGROUND

Restoration of the constitutional joint anatomy after hip replacement favours physiological peri-articular soft-tissue tension and kinematics, and is likely to be functionally beneficial. Hip resurfacing (HR) and conventional total hip replacement (THR) are two different options for replacing degenerated hips, and are likely to result in different anatomical reconstruction. We initiated this study to investigate the differences in gait performance between these two prosthetic options, and aimed to answer the following questions: (1) does HR result in better restoration of the frontal hip anatomical parameters, (2) and generate a more physiological gait compared to THR? (3) Does the quality of the anatomical restoration after THR influence gait performance?

HYPOTHESES

Our hypothesis was that a better anatomical restoration using HR versus THR would produce more physiological (symmetric) gait.

METHODS

We retrospectively reviewed 52 patients who had unilateral primary osteoarthritis successfully treated by replacement (40 THRs and 12 HRs). Hip anatomical parameters were measured on standing pelvic radiographs on both the prosthetic and the contralateral healthy hips. Patients undertook gait assessment under both normal and stress conditions at a mean follow-up of 14 months (7 to 16 months). Gait performances were compared between HR and THR, and the relationship between gait performances and quality of frontal anatomical restoration (estimated on radiograph) were assessed.

RESULTS

Compared to the native contralateral side, the HR procedure tended to decrease all independent anatomical radiographic parameters with the exception of the vertical centre of rotation offset, whilst the THR procedure tended to increase them; the difference between HR and THR was only statistically significant for femoral offset and global horizontal offset (increased after THR while reduced after HR). Only 50% of THR and 25% of HR procedures closely anatomically (±15%) recreated both global horizontal offset and global vertical offset. Under normal conditions (normal walking speed and flat ramp), the gait was fairly symmetric for both the HR and the THR patients with a symmetry index of 0.62% and 3.14% respectively. At high walking speed (stress conditions), the symmetry index degraded for both groups, but the gait remained more symmetric in the HR group (2.09%), compared to the THR group (5.74%); nevertheless, the difference remained not statistically significant (p=0.159). We were unable to detect any significant relationship between gait performances and radiographically measured hip frontal anatomical parameters.

DISCUSSION/CONCLUSIONS

HR procedure is more consistent than conventional THR in generating a more physiological gait under stress conditions. Radiographic estimation of the quality of the frontal anatomical hip restoration is of poor value to predict gait performances of THR patients.

LEVEL OF EVIDENCE

III - retrospective case-control study with prospective data collection.

Does the femoral offset affect replacements? The results from a National Joint Registry

BACKGROUND

Femoral component offset influences the torque forces exerted on a femoral stem and may therefore adversely affect femoral component survival. This study investigated the influence of femoral component offset on revision rates for primary total hip replacements (THR) registered on the New Zealand Joint Registry (NZJR).

METHODS

There were 106,139 primary THRs registered, resulting in 4960 revisions for any cause. There were 46,242 THRs performed using the five commonest femoral components listed on the NZJR. A total of 41,100 were done for primary osteoarthritis of which 40,548 had all the offset information available for analysis. We defined low offset as < 42 mm, standard as 42-48 mm and high offset as > 48 mm offset and examined revision rates according to the reasons for revision. We performed survival analyses for both cemented and uncemented femoral components grouped by the different offsets.

RESULTS

The all-cause revision rate was 0.54/100 component years (cys). Stems with < 42 mm offset had a revision rate of 0.58/100 cys (mean 0.58; 95% confidence interval (CI) 0.53-0.63), 42-48 mm offset 0.47 (95% CI 0.43-0.52) and > 48 mm offset 0.67 (95% CI 0.57-0.79). There was no significant difference in all-cause revision rates between varying stem offsets in uncemented stems adjusting for age and gender. In cemented stems both high and low offset stems were more likely to be revised. Uncemented stems of all offsets were more likely to undergo revision for femoral fracture.

CONCLUSIONS

Femoral component offset affects the overall all-cause revision rate of the most commonly used cemented stem, but not uncemented stem designs. In cemented stems offset influences the rate of revision for loosening and periprosthetic fractures.

CT-based Navigation System Using a Patient-Specific Instrument for Femoral Component Positioning: An Experimental in vitro Study with a Sawbone Model

PURPOSE

The intraoperative version of the femoral component is usually determined by visual appraisal of the stem position relative to the distal femoral condylar axis. However, several studies have suggested that a surgeon's visual assessment of the stem position has a high probability of misinterpretation. We developed a computed tomography (CT)-based navigation system with a patient-specific instrument (PSI) capable of three-dimensional (3D) printing and investigated its accuracy and consistency in comparison to the conventional technique of visual assessment of the stem position.

MATERIALS AND METHODS

A CT scan of a femur sawbone model was performed, and pre-experimental planning was completed. We conducted 30 femoral neck osteotomies using the conventional technique and another 30 femoral neck osteotomies using the proposed technique. The femoral medullary canals were identified in both groups using a box chisel.

RESULTS

For the absolute deviation between the measured and planned values, the mean two-dimensional anteversions of the proposed and conventional techniques were 1.41° and 4.78°, while their mean 3D anteversions were 1.15° and 3.31°. The mean θ₁, θ₂, θ₃, and d, all of which are parameters for evaluating femoral neck osteotomy, were 2.93°, 1.96°, 5.29°, and 0.48 mm for the proposed technique and 4.26°, 3.17°, 4.43°, and 3.15 mm for the conventional technique, respectively.

CONCLUSION

The CT-based navigation system with PSI was more accurate and consistent than the conventional technique for assessment of stem position. Therefore, it can be used to reduce the frequency of incorrect assessments of the stem position among surgeons and to help with accurate determination of stem anteversion.

Biomechanical Robustness of a Contemporary Cementless Stem to Surgical Variation in Stem Size and Position

Successful designs of total hip replacement (THR) need to be robust to surgical variation in sizing and positioning of the femoral stem. This study presents an automated method for comprehensive evaluation of the potential impact of surgical variability in sizing and positioning on the primary stability of a contemporary cementless femoral stem (Corail®, DePuy Synthes). A patient-specific finite element (FE) model of a femur was generated from computed tomography (CT) images from a female donor. An automated algorithm was developed to span the plausible surgical envelope of implant positions constrained by the inner cortical boundary. The analysis was performed on four stem sizes: oversized, ideal (nominal) sized, and undersized by up to two stem sizes. For each size, Latin hypercube sampling was used to generate models for 100 unique alignment scenarios. For each scenario, peak hip contact and muscle forces published for stair climbing were scaled to the donor's body weight and applied to the model. The risk of implant loosening was assessed by comparing the bone–implant micromotion/strains to thresholds (150 μm and 7000 με) above which fibrous tissue is expected to prevail and the periprosthetic bone to yield, respectively. The risk of long-term loosening due to adverse bone resorption was assessed using bone adaptation theory. The range of implant positions generated effectively spanned the available intracortical space. The Corail stem was found stable and robust to changes in size and position, with the majority of the bone–implant interface undergoing micromotion and interfacial strains that are well below 150 μm and 7000 με, respectively. Nevertheless, the range of implant positions generated caused an increase of up to 50% in peak micromotion and up to 25% in interfacial strains, particularly for retroverted stems placed in a medial position.

Geometric analysis and clinical outcome of two cemented stems for primary total hip replacement with and without modular necks

INTRODUCTION

Restoration of the physiological biomechanical principles of the hip is crucial in total hip replacement. The aim of this study was to compare an arthroplasty system with different offset options (a: Exeter^®) with a dual-modular stem (b: Profemur Xm^®).

MATERIALS AND METHODS

A local and an inertial coordinate system were used to assist the description of the components' assembly in the prosthesis. A resection line of the femoral head in standard position was added to the arthroplasties and geometric parameters were measured. The outcomes of 93 patients were clinically evaluated (a: n = 50, b: n = 43). Preoperative planning was compared to postoperative radiographs (femoral offset, leg-length), and clinical scores (HHS, WOMAC, total range of motion) were assessed preoperatively, and then 1 and 2 years after surgery.

RESULTS

The Exeter^® offers an offset range from 32.1 to 56.9 mm and the Profemur Xm^® a range from 29.3 to 55.3 mm. The leg-length variability of the Profemur Xm^® has a range of 25.9 mm, the Exeter^® a range of 13.7 mm. The Profemur Xm^® offers more possible combinations of offset and leg-length reconstruction. The neck-stem angles of the Exeter^® range from 125.2° to 126.3°, of the Profemur Xm^® from 127.2° to 142.6°. There was no statistically significant difference in clinical outcome and radiological parameters.

CONCLUSIONS

We conclude that both stems offer a wide range of options for anatomical reconstruction of the hip resulting in similarly good clinical results. The Profemur Xm^® stem has advantages for the reconstruction of hips that deviate from standard anatomy but has the drawback of additional corrosive wear at the stem/neck interface.

The History of Biomechanics in Total Hip Arthroplasty

Biomechanics of the hip joint describes how the complex combination of osseous, ligamentous, and muscular structures transfers the weight of the body from the axial skeleton into the appendicular skeleton of the lower limbs. Throughout history, several biomechanical studies based on theoretical mathematics, in vitro, in vivo as well as in silico models have been successfully performed. The insights gained from these studies have improved our understanding of the development of mechanical hip pathologies such as osteoarthritis, hip fractures, and developmental dysplasia of the hip. The main treatment of end-stage degeneration of the hip is total hip arthroplasty (THA). The increasing number of patients undergoing this surgical procedure, as well as their demand for more than just pain relief and leading an active lifestyle, has challenged surgeons and implant manufacturers to deliver higher function as well as longevity with the prosthesis. The science of biomechanics has played and will continue to play a crucial and integral role in achieving these goals. The aim of this article, therefore, is to present to the readers the key concepts in biomechanics of the hip and their application to THA.

The Femoral Head Center Shifts in a Mediocaudal Direction During Aging

BACKGROUND

Accurate reconstruction of hip joint biomechanics is the key stone in total hip arthroplasty. Although proximal femur morphology is known to vary with both age and gender, few studies investigated this in the very elderly (ie, ≥80 years). The purpose of this study was to compare basic morphological parameters describing the position of the femoral head between very elderly and middle-aged subjects.

METHODS

Computed tomographic scans of the femur of 90 very elderly (mean 84 years, range 80-105 years) and 58 middle-aged subjects (mean 52 years, range 20-79 years) were made. After 3-dimensional reconstruction, the neck-shaft angle, femoral neck anteversion angle, femoral head height, femoral neck length, and mediolateral offset (ML-offset) were determined.

RESULTS

The neck-shaft angle was on average 3.6° less in elderly males (125.9° ± 5.0°) than in middle-aged males (129.5° ± 5.1°) (P < .001). The femoral neck anteversion angle was not significantly different between both age and gender groups. The femoral head height was -12.3 mm in elderly females compared to -10.5 mm (Δ 17%) in elderly males (P = .284) and even -8.0 mm (Δ 54%) in middle-aged males. The ML-offset was 10% (4.1 mm) larger in elderly compared to middle-aged males (P < .001).

CONCLUSION

These findings suggest that the femoral neck and head shift in a relative varus position during aging. Femoral prostheses with increased ML-offset and a lower caput-collum-diaphyseal angle are needed to accommodate the morphology of the femur in the very elderly. Care must be taken not to lengthen the operated leg, particularly in very elderly females.

Selecting boundary conditions in physiological strain analysis of the femur: Balanced loads, inertia relief method and follower load

Selection of boundary constraints may influence amount and distribution of loads. The purpose of this study is to analyze the potential of inertia relief and follower load to maintain the effects of musculoskeletal loads even under large deflections in patient specific finite element models of intact or fractured bone compared to empiric boundary constraints which have been shown to lead to physiological displacements and surface strains. The goal is to elucidate the use of boundary conditions in strain analyses of bones. Finite element models of the intact femur and a model of clinically relevant fracture stabilization by locking plate fixation were analyzed with normal walking loading conditions for different boundary conditions, specifically re-balanced loading, inertia relief and follower load. Peak principal cortex surface strains for different boundary conditions are consistent (maximum deviation 13.7%) except for inertia relief without force balancing (maximum deviation 108.4%). Influence of follower load on displacements increases with higher deflection in fracture model (from 3% to 7% for force balanced model). For load balanced models, follower load had only minor influence, though the effect increases strongly with higher deflection. Conventional constraints of fixed nodes in space should be carefully reconsidered because their type and position are challenging to justify and for their potential to introduce relevant non-physiological reaction forces. Inertia relief provides an alternative method which yields physiological strain results.

Does component alignment affect gait symmetry in unilateral total hip arthroplasty patients?

BACKGROUND

Component malposition in total hip arthroplasty patients has been associated with adverse clinical outcomes. However, whether the component alignment influences hip dynamic performance following total hip arthroplasty remains unclear. The purpose of this study was to investigate the relationship between the component alignment and in vivo hip kinematics during gait.

METHODS

Nineteen unilateral total hip arthroplasty patients received CT scan for creation of 3D hip models. The component alignment between the non-implanted and implanted hips were measured and compared. Three-dimensional hip kinematics for both hips of the total hip arthroplasty patients during gait was quantified using a dual fluoroscopic imaging technique. The differences between the implanted and non-implanted hip kinematics during gait were calculated. A forward stepwise multiple linear regression was performed to evaluate the relationships between the changes in implanted hip kinematics and the differences in component alignment with respect to the non-implanted hips.

FINDINGS

An average 5.1° (SD 6.5°; range -11.1° to 18.3°) increase in internal rotation was observed in the implanted hip than the contralateral non-implanted hip and significantly correlated with a linear combination of the increase of cup anteversion, cup medial translation and leg lengthening (R=0.81).

INTERPRETATION

Results suggested that the total hip arthroplasty patients compensated the changes in hip geometry by altering the dynamic movement during gait. Restoration of the native hip geometry, including acetabular cup anteversion, position and leg length could be one of the factors that influence the hip kinematics symmetry in total hip arthroplasty patients during gait.

A Useful Anatomical Reference Guide for Stem Anteversion during Total Hip Arthroplasty in the Dysplastic Hip

Computed tomography scans of 50 dysplastic hips were obtained and reconstructed using preoperative planning software for total hip arthroplasty. The anteversion of the stem implanted parallel to the line connecting the trochanteric fossa and the middle of the medial cortex of the femoral neck (T line) was measured. The cutting heights of 5mm and 10mm above the lesser trochanter were simulated. The mean difference of the anteversion of the stem using the T line and the native femoral anteversion was 2.7° (95% CI: 1.0°-4.5°) and 3.5° (95% CI: 1.5°-5.5°) at cutting heights of 5mm and 10mm respectively. An anteversion using a T line is compatible with native femoral anteversion even in developmental dysplasia of the hip.

Reproducibility of femoral offset following short stem and straight stem total hip arthroplasty

Bone stock-preserving short stem prostheses have gained importance in total hip arthroplasty (THA) with the use of minimally invasive surgical procedures. Because of their metaphyseal fixation and their dependency on the calcar radius, it is unknown whether the femoral offset can be reproduced with the same accuracy following short stem vs standard stem THA. This study clarifies whether it is possible to restore the femoral offset using a short stem prosthesis (Fitmore; Zimmer, Warsaw, Indiana) compared with a conventional straight stem prosthesis (CLS; Zimmer) following minimally invasive implantation using an anterolateral approach. In a prospective, randomized, double-blinded study, 80 patients underwent THA using a short stem (SS group; n=40) or CLS implant (control group; n=40). Follow-up examinations were conducted 6 weeks postoperatively. Radiological and functional outcomes were measured. Subjective assessment of quality of life was evaluated using the Harris Hip Score (HHS), the Short Form 36-item health survey (SF-36), and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Pre- and postoperative comparison of the groups' change in offset showed no significant differences (SS group difference from pre- to postoperative, 6.1±6.5 mm; control group difference from pre- to postoperative, 6.5±7.1 mm; P=.93). Group comparison after 6 weeks revealed no significant differences in HHS, SF-36, or WOMAC. Based on these data, an equivalent reproducibility of the femoral offset was demonstrated following short stem and straight stem THA using a minimally invasive anterolateral approach. If the long-term results of short stems show a comparable survival, they represent a sensible alternative to standard stems.

Predicting the underestimation of the femoral offset in anteroposterior radiographs of the pelvis using 'lesser trochanter index': a 3D CT derived simulated radiographic analysis

We developed 'lesser trochanter index' (LTI) and estimated its accuracy in predicting the underestimation of offset in the anteroposterior (AP) pelvic radiographs. We reconstructed 320 simulated radiographs from the CT scans of 40 adult hips at different rotational projections of 10° increments from 30° internal rotation to 40° external rotation. Underestimation of femoral offset as a percentage was derived from the neck profile angle for all radiographs. Radiographs with an LTI value above 35 were 94% (95% CI, 89%-97%) likely to underestimate femoral offset by more than 5%. Radiographs with LTI between 0 and 30 demonstrated femoral offset within 5% of the true offset (predictive value 100%, CI 87%-100%). LTI could be a useful guide in preoperative templating of hip arthroplasty.

The importance of adequate stem anteversion for rotational stability in cemented total hip replacement

Progressive retroversion of a cemented stem is predictive of early loosening and failure. We assessed the relationship between direct post-operative stem anteversion, measured with CT, and the resulting rotational stability, measured with repeated radiostereometric analysis over ten years. The study comprised 60 cemented total hip replacements using one of two types of matt collared stem with a rounded cross-section. The patients were divided into three groups depending on their measured post-operative anteversion (< 10°, 10° to 25°, >  25°). There was a strong correlation between direct post-operative anteversion and later posterior rotation. At one year the < 10° group showed significantly more progressive retroversion together with distal migration, and this persisted to the ten-year follow-up. In the < 10° group four of ten stems (40%) had been revised at ten years, and an additional two stems (20%) were radiologically loose. In the ‘normal’ (10° to 25°) anteversion group there was one revised (3%) and one loose stem (3%) of a total of 30 stems, and in the > 25° group one stem (5%) was revised and another loose (5%) out of 20 stems. This poor outcome is partly dependent on the design of this prosthesis, but the results strongly suggest that the initial rotational position of cemented stems during surgery affects the subsequent progressive retroversion, subsidence and eventual loosening. The degree of retroversion may be sensitive to prosthetic design and stem size, but < 10° of anteversion appears deleterious to the long-term outcome for cemented hip prosthetic stems.

Cite this article: Bone Joint J 2013;95-B:23–30.

Does Choice of Head Size and Neck Geometry Affect Stem Migration in Modular Large-Diameter Metal-on-Metal Total Hip Arthroplasty? A Preliminary Analysis

Due to their theoretical advantages, hip systems combining modular necks and large diameter femoral heads have gradually gained popularity. However, among others, concerns regarding changes in the load transfer patterns were raised. Recent stress analyses have indeed shown that the use of modular necks and big femoral heads causes significant changes in the strain distribution along the femur. Our original hypothesis was that these changes may affect early distal migration of a modular stem. We examined the effect of head diameter and neck geometry on migration at two years of follow-up in a case series of 116 patients (125 hips), who have undergone primary Metal-on-Metal total hip arthroplasty with the modular grit-blasted Profemur®E stem combined with large-diameter heads (>36 mm). We found that choice of neck geometry and head diameter has no effect on stem migration. A multivariate regression analysis including the potential confounding variables of the body mass index, bone quality, canal fill and stem positioning revealed only a negative correlation between subsidence and canal fill in midstem area. Statistical analysis, despite its limitations, did not confirm our hypothesis that choice of neck geometry and/or head diameter affects early distal migration of a modular stem. However, the importance of correct stem sizing was revealed.

Femoral Offset (3D) in Patients without Osteoarthritis - Index Values from 200 Hip Joints

Introduction:

Femoral offset (FO) is a crucial parameter for hip joint biomechanics. Reference values for FO are particularly important when joint geometry has to be reconstructed during surgical interventions. Such reference values are scarce in literature and have mainly been obtained from osteoarthritis (OA) patients. The aim of this study was to conduct a patient-specific study of FO without osteoarthritis and to create a dataset of FO index values.

Materials and Methodology:

One hundred (49 female, 51 male; mean age: 59 (18 - 83) years) pelvic computed tomography (CT) scans were analyzed to determine FO in each patient. Bilateral symmetry and correlation between demographic data and FO were analyzed.

Results:

The mean FO ± SD was different for male (4.36 ± 0.56 cm) and female patients (3.95 ± 0.35 cm) (p <.0001). No Side differences of FO were observed in male and female patients. Significant correlation between height and FO was only observed in male patients.

Conclusion:

The values obtained in this study can be used as index values for the restoration and evaluation of hip geometry. For men, FO can be approximated using the correlation between FO and height.

The difference between stretching and splitting muscle trauma during THA seems not to play a dominant role in influencing periprosthetic BMD changes

BACKGROUND

Periprosthetic bone adaptation in the proximal femur after total hip arthroplasty can result in reduced bone mineral density that may contribute to increased risk of aseptic loosening or fracture. Functional loading of the proximal femur postoperatively may depend upon the type of surgical muscle trauma - splitting or stretching - and is likely to influence the preservation of periprosthetic bone mineral. Since the maintenance of bone is known to be highly age and gender dependent, the aim of this study was to investigate the interplay between muscle trauma and age and gender influences on periprosthetic bone adaptation.

METHODS

Ninet y-three patients were consecutively recruited into either a transgluteal (splitting) or anterolateral (stretching) surgical approach and examined 7 days and 12 months after an elective primary hip arthroplasty (Zweymüller Alloclassic stem), using dual-energy X-ray absorptiometry measurements to quantify proximal femoral bone mineral density.

FINDINGS

The results indicate that neither gender, age nor surgical trauma type, but only the combination of age and gender, were significant predictors of postoperative remodelling rate, with younger men (<65) and older women exhibiting the largest bone atrophy.

INTERPRETATION

This study has demonstrated that the difference between stretching and splitting surgical trauma to the muscles during total hip replacement does not play a dominant role in influencing periprosthetic bone mineral changes. However, this data does suggest that certain patient populations may particularly benefit from muscle and bone preserving procedures.

The accuracy of the use of functional hip motions on localization of the center of the hip

BACKGROUND

The hip joint is generally considered a ball-and-socket joint, the center of which is used as an anatomic landmark in functional analyses and by surgical navigation systems. The location of the hip center has been estimated using functional techniques using various limb motions. However, it is not clear which specific motions best predicted the functional center.

PURPOSE

This study aims to compare the predicted functional center of the hip evaluated from multiplanar circumduction and star motions, and to compare this functional center with the geometric center.

METHODS

Eight hips in four fresh-frozen cadavers were used and verified as morphologically normal in CT scans. Three-dimensional motion of each lower limb was recorded using arrays of reflective markers rigidly attached to the femur and pelvis. Each hip was manipulated to produce circumduction or star motion, i.e., abduction-adduction and flexion extension. The hip was then dissected and the bearing surface traced with a probe, from which a best-fit sphere was calculated. The functional center was calculated from the motion data and compared to the geometric technique.

RESULTS

There was no difference between the functional hip center predicted by circumduction or star motions, although this was offset from the geometric hip center by up to 14 mm. For all except two hips, the functional center was less than 6 mm from the geometric hip in each anatomic direction. Test-retest differences were smaller for circumduction than for star motions.

CONCLUSIONS

Estimation of the hip center based on motion of the femur relative to the pelvis could localize the geometric center of the joint within 14 mm and circumduction motions were more repeatable.

CLINICAL RELEVANCE

Many surgical navigation systems make use of the functional hip center as a landmark for alignment or reconstruction. Errors associated with this would have a very minor influence in lower limb alignment, e.g., for knee reconstruction, but could affect proximal femoral geometry relevant to hip reconstruction.

Toward a more realistic prediction of peri-prosthetic micromotions

The finite element (FE) method has become a common tool to evaluate peri-prosthetic micromotions in cementless total hip arthroplasty. Often, only the peak joint load and a selected number of muscle loads are applied to determine micromotions. Furthermore, the applied external constraints are simplified (diaphyseal fixation), resulting in a non-physiological situation. In this study, a scaled musculoskeletal model was used to extract a full set of muscle and hip joint loads occurring during a walking cycle. These loads were applied incrementally to an FE model to analyze micromotions. The relation between micromotions and external loads was investigated, and how micromotions during a full loading cycle compared to those calculated when applying a peak load only. Finally, the effect of external constraints was analyzed (full model vs. diaphyseal fixation and reduced number of muscle loads). Relatively large micromotions were found during the swing phase when the hip joint forces were relatively low. Maximal micromotions, however, did concur with the peak hip joint force. Applying only a peak joint force resulted in peak micromotions similar to those found when full walking cycle loads were applied. The magnitude and direction of the micromotions depended on the applied muscle loads, but not on external constraints.

Minimally invasive surgical approaches and traditional total hip arthroplasty: a meta-analysis of radiological and complications outcomes

Background

Minimally invasive total hip arthroplasty (MITHA) remains considerably controversial. Limited visibility and prosthesis malposition increase the risk of post-surgical complications compared to those of the traditional method.

Methods

A meta-analysis was undertaken of all published databases up to May 2011. The studies were divided into four subgroups according to the surgical approach taken. The radiological outcomes and complications of minimally invasive surgery were compared to traditional total hip arthroplasty (TTHA) using risk ratio, mean difference, and standardized mean difference statistics.

Results

In five studies involving the posterolateral approach, no significant differences were found between the MITHA groups and the TTHA groups in the acetabular cup abduction angle (p = 0.41), acetabular anteversion (p = 0.96), and femoral prosthesis position (p = 0.83). However, the femoral offset was significantly increased (WMD = 3.00; 95% CI, 0.40–5.60; p = 0.02). Additionally, there were no significant differences among the complications in both the groups (dislocations, nerve injury, infection, deep vein thrombosis, proximal femoral fracture) and revision rate (p>0.05). In three studies involving the posterior approach, there were no significant differences in radiological outcomes or all other complications between MITHA or TTHA groups (p>0.05). Three studies involved anterolateral approach, while 2 studies used the lateral approach. However, the information from imaging and complications was not adequate for statistical analysis.

Conclusions

Posterior MITHA seems to be a safe surgical procedure, without the increased risk of post-operative complication rates and component malposition rates. The posterolateral approach THA may lead to increased femoral offset. The current data are not enough to reach a positive conclusion that lateral and anterolateral approaches will result in increased risks of adverse effects and complications at the prosthesis site.

Generic rules of mechano-regulation combined with subject specific loading conditions can explain bone adaptation after THA

Bone adaptation after total hip arthroplasty is associated with the change in internal load environment, and can result in compromised bone stock, which presents a considerable challenge should a revision procedure be required. Under the assumption of a generic mechano-regulatory algorithm for governing bone adaptation, the aim of this study was to understand the contribution of subject specific loading conditions towards explaining the local periprosthetic remodelling variations in patients.

CT scans of 3 consecutive THA patients were obtained and used for the construction of subject specific finite element models using verified musculoskeletal loading and physiological boundary conditions. Using either strain energy density or equivalent strain as mechano-transduction signals, predictions of bone adaptation were compared to DEXA derived BMD changes from 7 days to 12 months post-implantation. Individual changes in BMD of up to 33.6% were observed within the 12 month follow-up period, together with considerable inter-patient variability of up to 26%. Estimates of bone adaptation using equivalent strain and balanced loading conditions led to the best agreement with in vivo measured BMD, with RMS errors of only 3.9%, 7.3% and 7.3% for the individual subjects, compared to errors of over 10% when the loading conditions were simplified.

This study provides evidence that subject specific loading conditions and physiological boundary constraints are essential for explaining inter-patient variations in bone adaptation patterns. This improved knowledge of the rules governing the adaptation of bone following THA helps towards understanding the interplay between mechanics and biology for better identifying patients at risk of excessive or problematic periprosthetic bone atrophy.

The direct lateral approach: impact on gait patterns, foot progression angle and pain in comparison with a minimally invasive anterolateral approach

INTRODUCTION

Minimally invasive total hip arthroplasty has been successfully introduced in the past decade. Nevertheless, standard approaches such as the direct lateral approach are still commonly used in orthopaedic surgery due to easy handling, good intra-operative overview and low complication rates. However, a frequent occurrence of fatty atrophy within the anterior third of the gluteus medius muscle has been demonstrated when using the modified direct-lateral approach (mDL), which may be associated with a reduction in function, limitation of internal leg rotation, gait disorders and pain. The question addressed in this study is whether mDL-approach leads to unfavourable changes in foot progression angle (FPA), gait and to more postoperative pain compared with a minimally invasive anterolateral approach (ALMI).

METHODS

Thirty patients with primary osteoarthritis of the hip were recruited for this study. All subjects received an uncemented THA (Alloclassic-Zweymüller stem, Allofit Cup, FA Zimmer), 15 through an ALMI-approach and 15 via the mDL-approach. Gait analyses were performed both preoperatively and 3 months after surgery to measure FPA, step length, stance duration, cadence and walking speed. Additionally, the Harris-Hip Score, pain according to the visual analogue scale and the Trendelenburg sign were evaluated.

RESULTS

No influence of the surgical approach could be observed on the gait patterns or FPA. Furthermore, neither increased external rotation of the limb nor restriction of internal rotation during walking could be established. Pain and Harris-Hip Score did not diVer significantly between the two groups.

CONCLUSION

In comparison with an ALMI approach, the mDL approach did not lead to a change in FPA postoperatively. No detrimental effect could be found on the gait pattern or pain after surgery. Based on these measurements, the minimally invasive anterolateral approach did not appear to provide functional benefits in outcome over the mDL approach. Consequently, both surgical approaches seem to be equally applicable approaches with good to very good functional results.

Patellofemoral joint contact forces during activities with high knee flexion

The patellofemoral (PF) joint plays an essential role in knee function, but little is known about the in vivo loading conditions at the joint. We hypothesized that the forces at the PF joint exceed the tibiofemoral (TF) forces during activities with high knee flexion. Motion analysis was performed in two patients with telemetric knee implants during walking, stair climbing, sit-to-stand, and squat. TF and PF forces were calculated using a musculoskeletal model, which was validated against the simultaneously measured in vivo TF forces, with mean errors of 10% and 21% for the two subjects. The in vivo peak TF forces of 2.9-3.4 bodyweight (BW) varied little across activities, while the peak PF forces showed significant variability, ranging from less than 1 BW during walking to more than 3 BW during high flexion activities, exceeding the TF forces. Together with previous in vivo measurements at the hip and knee, the PF forces determined here provide evidence that peak forces across these joints reach values of around 3 BW during high flexion activities, also suggesting that the in vivo loading conditions at the knee can only be fully understood if the forces at the TF and the PF joints are considered together.

Biomechanical evaluation of different offset versions of a cementless hip prosthesis by 3-dimensional measurement of micromotions

BACKGROUND

Cementless hip prostheses with different offsets are frequently used to restore the rotation center of the hip. However, a rising offset is theoretically associated with a potential risk for increased interface stresses and early loosening.

METHODS

To assess this potential risk for cementless stems, the primary stability of the CLS Spotorno stem was examined with respect to three different femoral neck versions (125°, 135° and 145°) measuring 3-dimensional micromotions. For this purpose 18 stems were implanted in composite femurs and tested dynamically using physiological loading conditions considering the necessary adaptation according to the different offsets. Additionally the deformations at the surface of the composite femur were recorded to draw conclusions about the tendency for stress shielding.

FINDINGS

The micromotions of the different offset versions were not significantly different. The highest values were obtained at the tip of the stems, even exceeding the critical limit for osseous integration of 150μm. Compared to untreated composite femurs the alteration of the deformations at the surface remained relatively low. A significant difference was only observed in the ventral measurement points.

INTERPRETATION

According to the measured micromotions no offset version of the CLS Spotorno can be declared as superior. The assumption that the varus version is characterized by extended interface stresses could not be confirmed. Furthermore, it could be demonstrated that according to the principle of proximal load transfer of the CLS Spotorno stem an osseous integration of the distal part cannot be expected and that the risk for stress shielding appears to be relatively low.

Dynamic hip joint stiffness in individuals with total hip arthroplasty: relationships between hip impairments and dynamics of the other joints

BACKGROUND

Little is known about hip joint stiffness during walking (dynamic joint stiffness) and the effect of hip impairments on biomechanical alterations of other joints in patients with total hip arthroplasty.

METHODS

Twenty-four patients (mean age 61.7 years) who underwent unilateral (n=12) or bilateral total hip arthroplasty (n=12) and healthy subjects (n=12) were recruited. In addition to kinematic and kinetic variables, dynamic hip joint stiffness which was calculated as an angular coefficient of linear regression of the plot of the hip flexion moment vs. hip extension angle during the late stance of gait, was measured. Group differences were compared using one-way ANOVA and Tukey's post-hoc test, and relationships between primary hip impairments and secondary gait impairments were found using partial correlation coefficients adjusted for gait speed and stride length.

FINDINGS

Dynamic hip joint stiffness was 47% higher on the side with the more pronounced limp in patients with bilateral arthroplasty than in healthy controls. In the same patients, increased dynamic hip joint stiffness was significantly associated especially with increased ankle plantarflexion moment on the ipsilateral side. In patients with unilateral arthroplasty, decreased hip power was significantly related to increased ankle plantarflexor power, only on the non-operated side.

INTERPRETATION

We found that dynamic hip joint stiffness was an important factor in assessing relationships between hip impairments and dynamics in other joints, especially in patients with bilateral total hip arthroplasty. The effects of altering hip joint stiffness on gait biomechanics need to be explored.

Influence of prosthesis design and implantation technique on implant stresses after cementless revision THR

BACKGROUND

Femoral offset influences the forces at the hip and the implant stresses after revision THR. For extended bone defects, these forces may cause considerable bending moments within the implant, possibly leading to implant failure. This study investigates the influences of femoral anteversion and offset on stresses in the Wagner SL revision stem implant under varying extents of bone defect conditions.

METHODS

Wagner SL revision stems with standard (34 mm) and increased offset (44 mm) were virtually implanted in a model femur with bone defects of variable extent (Paprosky I to IIIb). Variations in surgical technique were simulated by implanting the stems each at 4° or 14° of anteversion. Muscle and joint contact forces were applied to the reconstruction and implant stresses were determined using finite element analyses.

RESULTS

Whilst increasing the implant's offset by 10 mm led to increased implant stresses (16.7% in peak tensile stresses), altering anteversion played a lesser role (5%). Generally, larger stresses were observed with reduced bone support: implant stresses increased by as much as 59% for a type IIIb defect. With increased offset, the maximum tensile stress was 225 MPa.

CONCLUSION

Although increased stresses were observed within the stem with larger offset and increased anteversion, these findings indicate that restoration of offset, key to restoring joint function, is unlikely to result in excessive implant stresses under routine activities if appropriate fixation can be achieved.

Time course of skeletal muscle regeneration after severe trauma

BACKGROUND AND PURPOSE

Animal models of skeletal muscle injury should be thoroughly described and should mimic the clinical situation. We established a model of a critical size crush injury of the soleus muscle in rats. The aim was to describe the time course of skeletal muscle regeneration using mechanical, histological, and magnetic resonance (MR) tomographic methods.

METHODS

Left soleus muscles of 36 Sprague-Dawley rats were crushed in situ in a standardized manner. We scanned the lower legs of 6 animals by 7-tesla MR one week, 4 weeks, and 8 weeks after trauma. Regeneration was evaluated at these times by in vivo measurement of muscle contraction forces after fast-twitch and tetanic stimulation (groups 1W, 4W, 8W; 6 per group). Histological and immunohistological analysis was performed and the amount of fibrosis within the injured muscles was determined histomorphologically.

RESULTS

MR signals of the traumatized soleus muscles showed a clear time course concerning microstructure and T1 and T2 signal intensity. Newly developed neural endplates and myotendinous junctions could be seen in the injured zones of the soleus. Tetanic force increased continuously, starting at 23% (SD 4) of the control side (p < 0.001) 1 week after trauma and recovering to 55% (SD 23) after 8 weeks. Fibrotic tissue occupied 40% (SD 4) of the traumatized muscles after the first week, decreased to approximately 25% after 4 weeks, and remained at this value until 8 weeks.

INTERPRETATION

At both the functional level and the morphological level, skeletal muscle regeneration follows a distinct time course. Our trauma model allows investigation of muscle regeneration after a standardized injury to muscle fibers.

Component positioning in primary total hip replacement: a prospective comparative study of two anterolateral approaches, minimally invasive versus gluteus medius hemimyotomy

INTRODUCTION

One factor of implant survivorship in total hip replacement (THR) is the quality of implant choice and positioning. The purported advantages of minimally invasive approaches are faster recovery, shorter hospital stay and less per-operative blood loss. On the other hand, there have been many reports of higher complication rates, and doubts as to the quality of implant positioning.

HYPOTHESIS

The quest to minimize tissue damage is at the cost of THR positioning quality.

OBJECTIVES

To assess implant positioning in a prospective comparative continuous multicenter series.

PATIENTS AND METHODS

Between 2008 and 2009, a prospective comparative study was conducted on a continuous series of 141 THRs. Ninety-two were performed in two centers, using a minimally invasive Watson-Jones approach; the other 49, performed in a 3rd center, used an anterolateral approach with anterior hemimyotomy. The surgeons were in all cases experienced in their technique. Short-term follow-up comprised clinical and functional (Postel Merle d'Aubigné (PMA), Harris, SF12, WOMAC) and biological assessment (serum creatine phosphokinase (CPK), myoglobinemia, hematocrit) and analysis of complications and of implant positioning on X-ray and CT-scan.

RESULTS

On the Watson-Jones approach, surgery time was longer; day-1 analgesic administration was lower; PMA, Harris and WOMAC scores were better at 6 weeks; and CPK levels were lower at 24 and 48hours. There were no significant differences on the other clinical and biological criteria. Implant positioning analysis revealed significantly greater combined anteversion and greater variation in acetabular inclination mean with the Watson-Jones approach, but no differences in cup positioning, femoral stem positioning, or limb length discrepancy.

DISCUSSION

The minimally invasive Watson-Jones approach provided faster recovery and less muscular damage. However, implant positioning was less precise in terms of acetabular cup inclination.

LEVEL OF EVIDENCE

Level III. Prospective, comparative, non-randomized.

Lower limb biomechanics during gait do not return to normal following total hip arthroplasty

Although total hip arthroplasty (THA) is known to be a successful surgical procedure to alleviate hip pain and to improve health-related quality of life, these outcome measures in THA patients do not reach those of the general population. As a result, several investigators have assessed THA patients' gait mechanics, but most of them have ignored adjacent joints, as well as the effect that THA may have on the non-operated limb. The purpose of this investigation was to determine the effect of THA on the pelvis, hip, knee and ankle joint kinematics, as well as the hip, knee and ankle kinetics of both the operated and non-operated limbs during walking. These data were recorded for 20 patients having undergone unilateral THA and 20 healthy, matched control participants. Results revealed that the gait mechanics of THA patients did not return to normal 10.6 months, on average (+/-2.6 mo), following surgery. THA patients walked with lower operated-hip abduction moments, sagittal-plane range of motion, as well as lower generated and absorbed power, that may be consequential to pain-avoidance strategies adopted pre-operatively or to apprehensions associated with their new prosthesis. They also displayed various kinematic adaptations at the ankle joint of the operated limb and at the non-operated hip joint that may be leaving them at risk of developing other joint diseases. Further investigation is needed to confirm the reasons why THA patients' gait mechanics do not return to normal following surgery to develop better surgical techniques and/or rehabilitation programs.

Computer simulation of humeral shaft fracture in throwing

HYPOTHESIS

Throwing fractures of the humerus are well known, but the exact mechanism of injury is not clear. It has been postulated that these may be stress fractures because the forces have not seemed sufficient to cause acute fractures while throwing.

MATERIALS AND METHODS

Using finite element analysis, we reproduced fractures of the humerus using 3-dimensional models built from computed tomography images of 5 healthy volunteers. To apply the load during throwing, we assumed that the humeral head was completely fixed, and external rotation torque was applied to the distal end of the humerus until the stress of the bone elements became greater than yield stress. We reproduced the fracture line by removing the bone elements.

RESULTS

The maximum stress concentration was seen in the distal shaft, where a typical spiral fracture was created in all cases. In the humeral models, the torque required to initiate fracture ranged from 51 to 70 Nm. A strong correlation existed between the torque required to initiate fracture and thickness of the humeral cortical bone (R(2) = 0.74).

CONCLUSION

These results indicate that thickness of the humerus represents one factor contributing to fractures that occur while throwing.

LEVEL OF EVIDENCE

Basic science study.

Effect of modular neck variation on bone and cement mantle mechanics around a total hip arthroplasty stem

BACKGROUND

Total hip arthroplasty carried out using cemented modular-neck implants provides the surgeon with greater intra-operative flexibility and allows more controlled stem positioning.

METHODS

In this study, finite element models of a whole femur implanted with either the Exeter or with anew cemented modular-neck total hip arthroplasty (separate, neck and stem components) were developed.The changes in bone and cement mantle stress/strain were assessed for varying amounts of neck offset and version angle for the modular-neck device for two simulated physiological load cases: walking and stair climbing. Since the Exeter is the gold standard for polished cemented total hip arthroplasty stem design, bone and cement mantle stresses/strains in the modular-neck finite element models were compared with finite element results for the Exeter.

FINDINGS

For the two physiological load cases, stresses and strains in the bone and cement mantle were similar for all modular-neck geometries. These results were comparable to the bone and cement mechanics surrounding the Exeter. These findings suggest that the Exeter and the modular neck device distribute stress to the surrounding bone and cement in a similar manner.

INTERPRETATION

It is anticipated that the modular-neck device will have a similar short-term clinical performance to that of the Exeter, with the additional advantages of increased modularity.

Inaccuracies in the assessment of femoral anteversion in proximal femoral replacement prostheses

Accurately judging anteversion of proximal femoral replacement prostheses (PFRPs) during insertion can be difficult and may affect performance/stability. We designed a gravity goniometer jig, which attaches directly to the prosthesis, to improve rotational placement. Our hypotheses were that surgeons cannot accurately estimate anteversion in the context of a PFRP and that our jig would improve rotational placement. In our model, a 15-centimeter PFRP was placed in a saw-bones femur set in a lateral position. The prosthesis could rotate into anteversion/retroversion while the femur was fixed. Orthopedic residents and surgeons made 2 attempts to place the prosthesis into 15 degrees of anteversion; first without, then with the jig. Each attempt was recorded by photograph and angle-measuring jig. Nineteen surgeons and 28 residents participated. Without the jig, the mean PFRP anteversion was 18.3 degrees (range, 1 degrees -50.5 degrees ; SD, 9.6) compared to 14.3 degrees (range, 12.5 degrees -15.5 degrees ; SD, 0.7) using the jig (P = .006). Without a jig, the mean anteversion of resident-placed endoprosthesis was 18.4 degrees (range, 3 degrees -36 degrees ; SD, 8.1) compared to 18.1 degrees (range, 1 degrees -50.5 degrees ; SD, 12.1) (P = .91) recorded for attending physicians. The angle measurements from the jig and photographs were statistically equal. The placement of the PFRP into proper anteversion was more accurate with the aid of the jig. Simple jigs, such as the one developed for this study, may improve rotational placement of PFRPs.

[Musculoskeletal load analysis. A biomechanical explanation for clinical results--and more?]

Mechanical loading of the lower extremities due to muscle and joint contact forces plays an important role in orthopaedic and trauma surgery. Detailed, patient specific information on the in vivo forces and their distribution is, however, currently not readily available to the surgeon in clinical routine. The goal of this study was to elucidate the relationship between the position of the cup and the musculoskeletal loading conditions at the hip using validated analyses, and further, to evaluate the predictions of the biomechanical conditions against the results of a clinical study. The results indicate that restoring the anatomical hip centre to its anatomical mediolateral position could help to reduce joint loads and add to the longevity of the reconstruction. The routine use of validated analyses of musculoskeletal loading conditions, such as in the presented example using standardised pre-operative planning and sound intra-operative decision support systems, could contribute to securing a high standard in patient treatment.

Association between contact hip stress and RSA-measured wear rates in total hip arthroplasties of 31 patients

BACKGROUND

The main concern in the long run of total hip replacements is aseptic loosening of the prosthesis. Optimization of the biomechanics of the hip joint is necessary for optimization of long-term success. A widely implementable tool to predict biomechanical consequences of preoperatively planned reconstructions still has to be developed. A potentially useful model to this purpose has been developed previously. The aim of this study is to quantify the association between the estimated hip joint contact force by this biomechanical model and RSA-measured wear rates in a clinical setting.

METHODS

Thirty-one patients with a total hip replacement were measured with RSA, the gold standard for clinical wear measurements. The reference examination was done within 1 week of the operation and the follow-up examinations were done at 1, 2 and 5 years. Conventional pelvic X-rays were taken on the same day. The contact stress distribution in the hip joint was determined by the computer program HIPSTRESS. The procedure for the determination of the hip joint contact stress distribution is based on the mathematical model of the resultant hip force in the one-legged stance and the mathematical model of the contact stress distribution. The model for the force requires as input data, several geometrical parameters of the hip and the body weight, while the model for stress requires as input data, the magnitude and direction of the resultant hip force. The stress distribution is presented by the peak stress-the maximal value of stress on the weight-bearing area (p(max)) and also by the peak stress calculated with respect to the body weight (p(max)/W(B)) which gives the effect of hip geometry. Visualization of the relations between predicted values by the model and the wear at different points in the follow-up was done using scatterplots. Correlations were expressed as Pearson r values.

RESULTS

The predicted p(max) and wear were clearly correlated in the first year post-operatively (r = 0.58, p = 0.002), while this correlation is weaker after 2 years (r = 0.19, p = 0.337) and 5 years (r = 0.24, p = 0.235). The wear values at 1, 2 and 5 years post-operatively correlate with each other in the way that is expected considering the wear velocity curve of the whole group. The correlation between the predicted p(max) values of two observers who were blinded for each other's results was very good (r = 0.93, p < 0.001).

CONCLUSION

We conclude that the biomechanical model used in this paper provides a scientific foundation for the development of a new way of constructing preoperative biomechanical plans for total hip replacements.

Influence of changes in stem positioning on femoral loading after THR using a short-stemmed hip implant

BACKGROUND

Short-stemmed hip implants were introduced to conserve proximal bone mass and may facilitate the use of minimally invasive surgery, in which smaller incisions limit access to the joint. This limited access may increase the risk of surgical mal-positioning of the implant, however the sensitivity of femoral loading to such mal-positioning of a short-stemmed implant has not been studied.

METHODS

Finite element models were developed of a femur and a short-stemmed implant positioned to reproduce the intact hip centre, as well as with the implant placed in increased anteversion or offset. The effect of these surgical variables on femoral loading was examined for walking and stair climbing using loads from a validated musculoskeletal model. Results of the implanted models were compared with an intact model to evaluate stress shielding.

FINDINGS

Implant position had little influence on cortical strains along the length of the diaphysis, although strains decreased by up to 95% at the neck resection level compared to the intact femur. In the proximal Gruen zones I and VII strain energy density among the implanted models varied by up to 0.4 kJ/m(3) (28%) and 0.6 kJ/m(3) (24%) under walking and stair climbing, respectively. All implanted models showed characteristic proximal stress shielding, indicated by a decrease in strain energy density of up to 5.4 kJ/m(3) (69%) compared to the intact femur.

INTERPRETATION

Small changes in stem placement would likely have little influence on the internal loading of the femur after bone ingrowth has been achieved, however a reduction in strain energy density and therefore stress shielding was seen even for a short-stemmed implant, which may have consequences for longer-term bone remodelling.

Changes in femur stress after hip resurfacing arthroplasty: response to physiological loads

BACKGROUND

Hip resurfacing arthroplasty is being increasingly considered as an alternative to total hip arthroplasty in young, active patients. Hip resurfacing arthroplasty is reported to preserve the normal joint mechanics. However, there is concern, in the short term, due to frequent occurrence of femoral neck fractures.

METHODS

We evaluated changes in femoral mechanics after hip resurfacing arthroplasty. We used an experimentally validated, distributed material finite element model of a cadaveric femur before and after hip resurfacing arthroplasty. Bone stiffness and strength values representing normal, elderly and osteoporotic bone were used. For a physiological load case, bone strains were compared with literature values for total hip arthroplasty and a risk of fracture scalar calculated.

FINDINGS

The changes in peak stresses after hip resurfacing arthroplasty were low in relation to the failure strength of bone and the fracture risk was low. The intact and implanted finite element models showed bone strains after hip resurfacing arthroplasty were closer to the intact condition than after total hip arthroplasty.

INTERPRETATION

The bone stresses predicted after resurfacing in both the normal and aged femoral neck were not sufficient to be a potential cause of fracture.

Physiologically based boundary conditions in finite element modelling

Finite element analysis has been used extensively in the study of bone loading and implant performance, such as in the femur. The boundary conditions applied vary widely, generally producing excessive femoral deformation, and although it has been shown that the muscle forces influence femoral deflections and loading, little consideration has been given to the displacement constraints. It is hypothesised that careful application of physiologically based constraints can produce physiological deformation, and therefore straining, of the femur. Joint contact forces and a complete set of muscle forces were calculated based on the geometry of the Standardised Femur using previously validated musculoskeletal models. Five boundary condition cases were applied to a finite element model of the Standardised Femur: (A) diaphyseally constrained with hip contact and abductor forces; (B) case A plus vasti forces; (C) case A with complete set of muscle forces; (D) distally constrained with all muscle forces; (E) physiological constraints with all muscle forces. It was seen that only the physiological boundary conditions, case E, produced physiological deflections (< 2.0mm) of the femoral head in both the coronal and sagittal planes, which resulted in minimal reaction forces at the constrained nodes. Strains in the mid-diaphysis varied by up to 600 micro-strain under walking loads and 1000 micro-strain under stair climbing loads. The mode of loading, as indicated by the strain profiles on the cortex also varied substantially under these boundary conditions, which has important consequences for studies that examine localised bone loading such as fracture or bone remodelling simulations.