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

The press-fit technique without screws and bone graft can be used as an alternative method in Crowe type II and III hip dysplasia

Background/aim

Developmental dysplasia of the hip (DDH) is the most common cause of secondary hip arthrosis. The primary purpose of this study was to assess the results of an oversized hemispherical cup via the press-fit technique used for Crowe type II and III DDH without screws and to determine if adequate medialization and initial stability of the acetabular component would allow us to avoid screw and graft use.

Materials and methods

Between February 2012 and May 2020, the current study analyzed 43 hips with Crowe type II and III DDH treated with a porous-coated cup by placing the press-fit technique or screw. The acetabular cup was fixed with the press-fit technique without additional screws in 27 hips and with screws in 16 hips. The inclusion criterion in this study was a minimum 2-year-period after the surgery.

Results

The mean duration of follow-up was 6.83 ± 2.67 years in the press-fit group and 6.21 ± 2.01 years in the screw group. The mean age of the patients was 47.96 ± 12.37 years in the press-fit group and 50.5 ± 12.37 years in the screw group. Measurements revealed that the hip center of rotation (HCR) was located more medially and superiorly postoperatively than preoperatively in both groups. The mean cup coverage in the screw group was 85.58% ± 7.51% (75.3%-97.2%), while it was less than 90.41% ± 6.15% (76.3%-98.2%) in the press-fit group (p = 0.038). No component was revised because of loosening, and all of the implants were radiologically stable within the observation period. No statistically significant differences were observed regarding the postoperative limp-length discrepancy between the groups (p = 0.496).

Conclusion

Press-fit implantation of a porous-coated acetabular component without screws can also be used as an alternative method for THA in Crowe type II and III DDH. The initial stability was obtained using the press-fit technique with a small cup positioned more medially and superiorly, which may allow the surgeon to avoid screw and graft use.

Downsizing and minimising medialisation of the acetabular component: Novel technique to preserve bone in THA

Standard practice for acetabular component placement in total hip arthroplasty (THA) is to medialise the acetabular component. Bone preservation techniques during primary THA are beneficial for possible future revisions. The goal of this study is to examine the effect of downsizing and minimising medialisation of the acetabular component on bone resection volume. The volume of bone resected during acetabular preparation for different sizes of components was calculated and the volume of bone preserved by downsizing the cup was determined. Minimising medialisation of the acetabular component by 1-3 mm from the true floor was calculated. Absolute values and percentage of bone volume preserved when acetabular components are downsized or less medialised is presented. Downsizing the acetabular component by one size (2 mm) preserves between 2.6 cm³ (size 40 vs 42) and 8.4 cm³ (size 72 vs 74) of bone volume and consistently reduces resected bone volume by at least 35% (range 35.2%-37.5%). Similarly, reducing medialisation of a 56 mm acetabular cup (as an example of a commonly implanted component) by 3 mm reduces bone loss by 5.9 cm³- 44% less bone volume resection. Downsizing and minimising medialisation of the cup in THA substantially preserves bone which may benefit future revision surgeries. Surgeons could consider implanting the smallest acceptable acetabular shell to preserve bone without compromising on head size.

Clinical and Radiological Results of Crowe Type 3 or 4 Dysplasia Patients Operated on With Total Hip Arthroplasty Using a Cementless Rectangular Femoral Component Without Fixating or Grafting the Transverse Osteotomy Site

BACKGROUND

Total hip arthroplasty (THA) involving shortening osteotomy for patients with Crowe type 3 or 4 dysplasia is a challenging surgical procedure. This study aims to demonstrate that rectangular femoral component use in anatomical reconstructions with THA and transverse shortening osteotomy yields successful results without the use of bone graft or any fixation material at the osteotomy site.

METHODS

Fifty hips from 41 patients were identified retrospectively as per study objectives. All patients were evaluated using the Harris Hip Score and Visual Analog Scale for pain. Complications were reported. Radiological evaluation criteria were then evaluated, including leg length discrepancy, degree of trochanter caudalization and stem subsidence, radiolucent and radiodense lines for both components and bone atrophy or hypertrophy around the stem according to Gruen zones, and the canal fill ratio of the stem.

RESULTS

Postoperative Harris Hip Score was excellent for 68% of patients. No patient had poor results. Complication rate was 32%. One patient had nonunion (2%). The mean postoperative leg length discrepancy was 0.8(±0.6) cm. No patient had a subsidence of more than 5 mm. Radiolucent and radiodense lines were present in up to 34% of patients, and bone atrophy was present in the proximal femur in up to 96% of patients. No patient had osteolysis or loosening in neither component.

CONCLUSION

Successful clinical and radiological results can be obtained from Crowe type 3 and 4 dysplastic hips operated on with THA using a rectangular femoral component and transverse shortening osteotomy technique. The use of graft or any fixation material at the osteotomy site is not mandatory.

Evaluation and validation of 2D biomechanical models of the knee for radiograph-based preoperative planning in total knee arthroplasty

Thorough preoperative planning in total knee arthroplasty is essential to reduce implant failure by proper implant sizing and alignment. The “gold standard” in conventional preoperative planning is based on anterior-posterior long-leg radiographs. However, the coronal component alignment is still an open discussion in literature, since studies have reported contradictory outcomes on survivorship, indicating that optimal individual alignment goals still need to be defined. Two-dimensional biomechanical models of the knee have the potential to predict joint forces and, therefore, objectify therapy planning. Previously published two-dimensional biomechanical models were evaluated and validated for the first time in this study by comparison of model predictions to corresponding in vivo measurements obtained from telemetric implants for a one- and two-leg stance. Model input parameters were acquired from weight-bearing anterior-posterior long-leg radiographs and statistical assumptions for patient-specific model adaptation. The overall time from initialization to load prediction was in the range of 7–8 minutes per patient for all models. However, no model could accurately predict the correct trend of knee joint forces over patients. Two dimensional biomechanical models of the knee have the potential to improve preoperative planning in total knee arthroplasty by providing additional individual biomechanical information to the surgeon. Although integration into the clinical workflow might be performed with acceptable costs, the models’ accuracy is insufficient for the moment. Future work is needed for model optimization and more sophisticated modelling approaches.

ESB Clinical Biomechanics Award 2018: Muscle atrophy-related increased joint loading after total hip arthroplasty and their postoperative change from 3 to 50 months

BACKGROUND

Hip joint loading is dominated by muscular activity. Thus, contact forces exceeding many times one's body weight are a consequence of imbalanced muscular activity. The objective was to analyze the influence of muscle atrophy after total hip arthroplasty on in vivo hip joint contact loading initially and long term. We hypothesized that an impaired periarticular muscle will lead to increase in vivo joint load, specifically in the long term.

METHODS

Using a group of nine patients with instrumented hip implants, contact forces and muscle status were analyzed one day prior to 3 and 50 months after joint arthroplasty. In vivo load measurements were performed for different activities of daily living (ADL). Pre- and postoperative pelvic CT scans were analyzed to assess the periarticular muscle status. Finally, the muscle morphologies and in vivo contact forces were compared.

RESULTS

At 3 months after total hip arthroplasty we found a significant correlation of lower lean gluteus minimus muscle (GMin) volume with higher loads during all tested activities of daily living. 50 months postoperatively statistical analysis revealed lower lean volume of the gluteus maximus to be correlated with higher joint loads in walking.

CONCLUSION

Our data generally show a good comparability between muscle status and joint contact forces and thus support our hypothesis that an impairment of periarticular musculature contributes to an increase of the in vivo joint loads after total hip arthroplasty. Effects were most pronounced during stair climbing and sit-down/stand-up from a chair at 3 months and during level walking at 50 months.

The influence of cavity preparation and press-fit cup implantation on restoring the hip rotation center

BACKGROUND

Reaming of the acetabular cavity and cup implantation directly influence the hip rotation center and contact area between implant and bone. Previous studies have reported on an altered rotation center after total hip arthroplasty, but have not studied the influence of reaming and cup implantation separately. Aim of this study was therefore to analyze the individual influence of acetabular reaming and subsequent cup implantation on the rotation center and how this influences the contact conditions at the bone-implant interface.

METHODS

Acetabular press-fit cups were implanted into the left and right hips of three full cadavers (n = 6). CT scans were performed to calculate the change in hip rotation center after reaming and prior to liner insertion. 3D models of the cups were used to determine the polar gap, the contact conditions and the effective press-fit.

FINDINGS

Reaming the acetabular cavity shifted the rotation center medially (median 5.8 mm, range 4.8-9.1), superiorly (5.3 mm, 3.0-7.0) and posteriorly (2.9 mm, 1.0-5.3). With cup implantation, the rotation center shifted back towards the native position, but no full restoration was observed. The degree of shift increased with the size of polar gap (r_s = 0.829, P = .042), which inversely reduced the contact area (r_s = 0.886, P = .019).

INTERPRETATION

This study reveals that the dominant factor in hip rotation center restoration is the reaming process, while the cup implantation for a given nominal press-fit has only a small influence. Increasing the press-fit would improve the restoration but bares the danger of insufficient bone coverage and periprosthetic fractures due to the high forces needed.

[Biomechanical modeling and the relevance for total hip arthroplasty]

BACKGROUND

Providing the hip with an endoprosthesis is one of the most common orthopedic interventions in Germany. The long-term success of such a procedure depends on the consideration of the loads due to muscle and joint forces in the planning and operative care. Patient-specific information of forces acting in vivo is not available to the surgeon in clinical routine today. This is where biomechanical modeling comes in.

PROCEDURES

A field of activity of biomechanical modeling is the development of methods and procedures for the precise analysis and simulation of endoprosthetic supplies. The aim was to show the possibilities of biomechanical modeling in total hip arthroplasty by means of two examples (sensitivity analysis and pre-/postoperative comparison of intervention outcome).

RESULTS

The results of the sensitivity analysis showed that by modeling the position of an optimal reconstruction of the hip rotational center can be found and the forces acting on the hip joint minimized. In the case of the pre-/postoperative comparison, it can be analyzed whether there has been a decrease or increase of load postoperatively, respectively, or whether the conditions are considered to be approximately equal to the preoperative situation. In the future, biomechanical modeling will be able to significantly improve long-term function by reducing wear and optimizing muscular function of the joint. Therefore, the routine use of validated musculoskeletal analysis in the context of standardized preoperative planning and intraoperative navigation-based implementation should be considered. Thus, validated analyses of musculoskeletal loads not only contribute to the extension of basic knowledge but also to the optimization of endoprosthetic care through their integration into the clinical workflow.

Gluteal muscle damage leads to higher in vivo hip joint loads 3 months after total hip arthroplasty

Background

Total hip arthroplasty (THA) is in most cases improving patients´ life quality immediately after surgery. However, a closer look at these patients, especially with modern gait analysis methods, reveals also negative consequences due to the surgical approach related injury to the pelvic muscles. We hypothesized that this damage will have a negative impact on hip joint contact forces during activities of daily living (ADL).

Methods

10 patients undergoing THA received an instrumented hip joint implant enabling real time in vivo measurements of hip joint loads using a direct lateral approach. Pre- and 3 months postoperative computed tomography (CT) scans were used for evaluation of the periarticular muscle status, using muscle volume, fat ratio and lean muscle volume as parameters. An analysis of in vivo hip contact forces was made 3 months after THA during ADL (walking, stair climbing, chair rising and sitting) and correlated with the morphology of the periarticular muscles.

Results

We found a significant decrease of volume by 25% (-3 to -45, p = 0.005) and increase in fat ratio of the Gluteus Minimus (Gmin), resulting in a decrease in lean muscle volume of 28% (-48 to 0, p = 0.008). This was accompanied by an inverse development in the Tensor Fasciae Latae (TFL) resulting in a lean muscle volume increase of 34% (-2 to -102, p = 0.013). Changes in Gluteus Medius (Gmed) and Gluteus Maximus (Gmax) have not been observed in the short-term follow up. A decreased Gmin lean muscle volume was found to strongly correlate with high in vivo joint contact forces in all tested ADL.

Conclusion

The decrease of Gmin volume can be seen as a direct effect of THA surgery, whereas the increase of TFL might compensate for loss of Gmin volume. Lean muscle volume and fat ratio were better predictors for joint contact forces than total muscle volume. These effects were most pronounced during sitting down and standing up due to the higher demand on the gluteal muscles during these activities.

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.

Effect of the lateral rotators on load transfer in the human hip joint revealed by mechanical analysis

A deeper understanding of load transfer in the human hip joint is of great importance as there is strong evidence that the mechanical loading of the hip has a major effect on the onset and progression of osteoarthritis. In this work, a biomechanical model of the human hip joint is developed which takes the lateral rotators into account. On the basis of a two-dimensional analysis of the human hip joint, the dependencies of the hip joint reaction force and its angle to the vertical are derived. The dependencies can be given as explicit equations. In addition, a numerical finite element analysis has been set up to calculate the contact pressure distribution on the femoral head. The results of this study are not subject-specific and are intended to show qualitative results and relationships of the load transfer behavior. The results of this two-dimensional study show that the lateral rotators have a significant effect on the contact pressure distribution in the human hip joint. Activated lateral rotators shift the maximum contact pressure in the medial direction and the contact pressure at the lateral edge of the contact area is significantly reduced. The results are validated by comparison to results in the literature and subsequently discussed. The results give additional insight into the load transfer behavior of the human hip joint and might be of relevance to investigations on the development of conservative therapies for osteoarthritic hips.

Personalized neuromusculoskeletal modeling to improve treatment of mobility impairments: a perspective from European research sites

Mobility impairments due to injury or disease have a significant impact on quality of life. Consequently, development of effective treatments to restore or replace lost function is an important societal challenge. In current clinical practice, a treatment plan is often selected from a standard menu of options rather than customized to the unique characteristics of the patient. Furthermore, the treatment selection process is normally based on subjective clinical experience rather than objective prediction of post-treatment function. The net result is treatment methods that are less effective than desired at restoring lost function. This paper discusses the possible use of personalized neuromusculoskeletal computer models to improve customization, objectivity, and ultimately effectiveness of treatments for mobility impairments. The discussion is based on information gathered from academic and industrial research sites throughout Europe, and both clinical and technical aspects of personalized neuromusculoskeletal modeling are explored. On the clinical front, we discuss the purpose and process of personalized neuromusculoskeletal modeling, the application of personalized models to clinical problems, and gaps in clinical application. On the technical front, we discuss current capabilities of personalized neuromusculoskeletal models along with technical gaps that limit future clinical application. We conclude by summarizing recommendations for future research efforts that would allow personalized neuromusculoskeletal models to make the greatest impact possible on treatment design for mobility impairments.

Treatment for acetabular dysplasia using the uncemented RM acetabular component - a 20 year follow-up

Our study reports a sub-group of patients with developmental dysplasia from a previously published larger series, with particular emphasis on the use of the uncemented RM acetabular component. We evaluated the long term results of 93 consecutive uncemented THAs in 80 patients using the titanium-coated RM acetabular component and the CLS femoral component in a prospective study. Eighteen hips in 16 patients had osteoarthritis secondary to developmental dysplasia of the hip. The mean follow-up was 19.6 years (18.2 to 20.9). Fourteen patients with 15 hips were clinically and radiographically examined and evaluated. Two patients with 3 hips died. No patient was lost to follow-up. No implant had to be revised, and no cases showed evidence of radiographic loosening. Nine acetabular components were not completely covered by host bone but this did not affect the outcome. At the latest follow-up the mean Harris Hip Score was 92 (81 to 100). The mean annual wear rate was 0.12 mm. The RM acetabular component performed well over 20 years in this selected group of patients. Complete acetabular containment was not needed, thus allowing reliable reconstruction of the anatomical centre of rotation.

Femoral neck cut level affects positioning of modular short-stem implant

A trend in total hip arthroplasty surgery has been to design more bone-preserving procedures, especially for younger patients. This study investigated the final implant positioning of a short metaphyseal femoral neck type of implant to determine whether leg length, caput collum diaphysis (CCD) angle, and offset could be re-created with different levels of femoral neck resection. Ten cadaveric hips in 6 whole-body specimens were used, with 3 fiducial markers to allow registration of computer navigation points to computed tomography scan data. Three femoral neck resection levels were investigated: 0 mm, +5 mm (the recommended level of resection), and +10 mm from the base of the femoral neck. Results showed that the CCD angle was significantly higher with 0-mm neck cut and the offset was lower, whereas the highest neck cut had longer leg-length results. Surgeons who use a short metaphyseal stem need to realize the importance of a proper femoral neck cut to restore anatomic parameters as well as the possible benefit of computer-assisted surgery to restore these anatomic parameters during surgery.

Influence of the acetabular cup position on hip load during arthroplasty in hip dysplasia

Placement of the acetabular cup during total hip arthroplasty is of great importance because usually every deviation from the ideal centre of rotation negatively influences endoprosthesis survival, polyethylene wear and hip load. Here we present hip load change in respect to various acetabular cup positions in female patients who underwent total hip replacement surgery due to hip dysplasia. The calculation suggests that, in the majority of cases, for every millimeter of lateral displacement of the acetabular cup (relative to the ideal centre of rotation) an increase of 0.7% in hip load should be expected and for every millimeter of proximal displacement an increase of 0.1% in hip load should be expected (or decreased if displacement is medial or distal). Also, for every millimeter of neck length increase, 1% decrease is expected and for every millimeter of lateral offset, 0.8% decrease is expected. Altogether, hip load decreases when the cup is placed more medially or distally and when the femoral neck is longer or lateral offset is used.

Assessment of the primary rotational stability of uncemented hip stems using an analytical model: comparison with finite element analyses

Background

Sufficient primary stability is a prerequisite for the clinical success of cementless implants. Therefore, it is important to have an estimation of the primary stability that can be achieved with new stem designs in a pre-clinical trial. Fast assessment of the primary stability is also useful in the preoperative planning of total hip replacements, and to an even larger extent in intraoperatively custom-made prosthesis systems, which result in a wide variety of stem geometries.

Methods

An analytical model is proposed to numerically predict the relative primary stability of cementless hip stems. This analytical approach is based upon the principle of virtual work and a straightforward mechanical model. For five custom-made implant designs, the resistance against axial rotation was assessed through the analytical model as well as through finite element modelling (FEM).

Results

The analytical approach can be considered as a first attempt to theoretically evaluate the primary stability of hip stems without using FEM, which makes it fast and inexpensive compared to other methods. A reasonable agreement was found in the stability ranking of the stems obtained with both methods. However, due to the simplifying assumptions underlying the analytical model it predicts very rigid stability behaviour: estimated stem rotation was two to three orders of magnitude smaller, compared with the FEM results.

Conclusion

Based on the results of this study, the analytical model might be useful as a comparative tool for the assessment of the primary stability of cementless hip stems.