Superior displacement of the hip in total joint replacement: effects of prosthetic neck length, neck-stem angle, and anteversion angle on the moment-generating capacity of the muscles

Can the hip joint center be estimated from pelvic dimensions in dysplastic hips?

BACKGROUND

We aimed to determine correlations between the hip joint center position and pelvic dimensions and whether the three-dimensional position of the original hip joint center could be estimated from pelvic landmarks in dysplastic and normal hips.

METHODS

We reviewed the pelvic CT scans of 70 patients (70 hips) with hip dysplasia. Seventy-seven normal hips were used as controls. The hip joint center coordinates (Cx, Cy, and Cz) and pelvic dimensions were measured with reference to the anterior pelvic plane coordinate system. Multiple regression formulas were used to estimate the original hip joint center.

RESULTS

The hip center for both dysplastic and normal hip was highly correlated with the distance between the anterior superior iliac spine (ASIS) in the coronal plane (r = 0.76 and 0.84), the distance from the ASIS to the pubic tubercle in the sagittal plane (r = 0.81 and 0.76), and distance from the pubic tubercle to the most posterior point of the ischium on the transverse plane (r = 0.76 and 0.78). The hip joint center could be estimated within a 5-mm error for more than 80% of hips on their respective axes in both dysplastic and normal hips.

CONCLUSIONS

The three-dimensional position of the original hip joint center was correlated with pelvic dimensions, and can be estimated with substantial accuracy using pelvic landmarks as references. Although these results are preliminary, this estimation method may be useful for surgeons planning total hip arthroplasties.

Pseudoarthrosis of the ilium after periacetabular osteotomy that was treated by cemented total hip arthroplasty: a case report

Background

Preserving the hip joint to delay arthroplasty for patients with acetabular dysplasia-associated early-stage osteoarthritis has become more common, and several surgical procedures have demonstrated pain relief and improved hip joint function. Periacetabular osteotomy, one of the joint-preserving surgical procedures of the hip, provides favorable outcomes, although there are no reports of total hip arthroplasty being used to treat pseudoarthrosis of the periacetabular osteotomy segment. Therefore, we report a case of pseudoarthrosis in the osteotomy segment after periacetabular osteotomy. The patient was treated using modified total hip arthroplasty and achieved a favorable short-term outcome.

Case presentation

A 62-year-old Japanese woman was diagnosed with bilateral acetabular dysplasia at the age of 50 years, and underwent right and left periacetabular osteotomy at the ages of 52 and 55 years, respectively. When she was 61-years old, she experienced repeated episodes of left coxalgia during walking, with increasing pain at rest, and subsequently visited our department. Plain radiography and computed tomography of her left hip joint confirmed pseudoarthrosis of the periacetabular osteotomy segment. In addition, narrowing of her left hip joint space was observed, which indicated advanced osteoarthritis of the hip. Therefore, she underwent left total hip arthroplasty when she was 62-years old. During the surgery, fibrous fusion of the periacetabular osteotomy segment was confirmed via fluoroscopy, although no abnormal mobility was observed. Thus, the osteotomy segment was fixed with one absorbable screw and two bone pegs (which were prepared using allogeneic bone), and the acetabular cup was fixed using cement. Her postoperative course was generally favorable and bone fusion of the periacetabular osteotomy segment was confirmed at 3 years and 6 months after surgery. Her modified Harris hip score was 43 before the surgery and had improved to 90 at the final follow-up.

Conclusions

Modified total hip arthroplasty was successfully used to treat osteoarthritis of the hip and pseudoarthrosis of the periacetabular osteotomy segment. This procedure achieved pseudoarthrosis region bone fusion and a favorable postoperative outcome.

A Novel Approach for Dynamic Testing of Total Hip Dislocation under Physiological Conditions

Constant high rates of dislocation-related complications of total hip replacements (THRs) show that contributing factors like implant position and design, soft tissue condition and dynamics of physiological motions have not yet been fully understood. As in vivo measurements of excessive motions are not possible due to ethical objections, a comprehensive approach is proposed which is capable of testing THR stability under dynamic, reproducible and physiological conditions. The approach is based on a hardware-in-the-loop (HiL) simulation where a robotic physical setup interacts with a computational musculoskeletal model based on inverse dynamics. A major objective of this work was the validation of the HiL test system against in vivo data derived from patients with instrumented THRs. Moreover, the impact of certain test conditions, such as joint lubrication, implant position, load level in terms of body mass and removal of muscle structures, was evaluated within several HiL simulations. The outcomes for a normal sitting down and standing up maneuver revealed good agreement in trend and magnitude compared with in vivo measured hip joint forces. For a deep maneuver with femoral adduction, lubrication was shown to cause less friction torques than under dry conditions. Similarly, it could be demonstrated that less cup anteversion and inclination lead to earlier impingement in flexion motion including pelvic tilt for selected combinations of cup and stem positions. Reducing body mass did not influence impingement-free range of motion and dislocation behavior; however, higher resisting torques were observed under higher loads. Muscle removal emulating a posterior surgical approach indicated alterations in THR loading and the instability process in contrast to a reference case with intact musculature. Based on the presented data, it can be concluded that the HiL test system is able to reproduce comparable joint dynamics as present in THR patients.

Estimation of the hip joint centre in human motion analysis: a systematic review

BACKGROUND

Inaccuracies in locating the three-dimensional position of the hip joint centre affect the calculated hip and knee kinematics, force- and moment-generating capacity of muscles and hip joint mechanics, which can lead to incorrect interpretations and recommendations in gait analysis. Several functional and predictive methods have been developed to estimate the hip joint centre location, and the International Society of Biomechanics recommends a functional approach for use with participants that have adequate range of motion at the hip, and predictive methods in those with insufficient range of motion. The purpose of the current systematic review was to substantiate the International Society of Biomechanics recommendations. This included identifying the most accurate functional and predictive methods, and defining 'adequate' range of motion.

METHODS

A systematic search with broad search terms was performed including five databases.

FINDINGS

The systematic search yielded to 801 articles, of which 34 papers were included. Eleven different predictive and 13 different functional methods were identified. The results showed that the geometric sphere fit method and Harrington equations are the most accurate functional and predictive approaches respectively that have been evaluated in vivo.

INTERPRETATION

In regard to the International Society of Biomechanics recommendations, the geometric sphere fit method should be used in people with sufficient active hip range of motion and the Harrington equations should be used in patients without sufficient hip range of motion. Multi-plane movement trials with at least 60° of flexion-extension and 30° of ab-adduction range of motion are suggested when using functional methods.

Association of hip and pelvic geometry with tibiofemoral osteoarthritis: multicenter osteoarthritis study (MOST)

OBJECTIVE

Lateral tibiofemoral osteoarthritis (OA) is overall less common than medial tibiofemoral OA, but it is more prevalent in women. This may be explained by sex differences in hip and pelvic geometry. The aim of this study is to explore sex differences in hip and pelvic geometry and determine if such parameters are associated with the presence of compartment-specific knee OA.

METHODS

This case-control study reports on 1,328 hips/knees from 664 participants and is an ancillary to the Multicenter Osteoarthritis Study (MOST). Of the 1,328 knees, 219 had lateral OA, 260 medial OA, and 849 no OA. Hip and pelvic measurements were taken from full-limb radiographs on the ipsilateral side of the knee of interest. After adjusting for covariates, means were compared between sexes and also between knees with medial and lateral OA vs no OA using separate regression models.

RESULTS

Women were shown to have a reduced femoral offset (FO) (mean 40.9 mm vs 45.9 mm; P = 0.001) and more valgus neck-shaft angle (mean 128.4° vs 125.9°; P < 0.001) compared to men. Compared to those with no OA, knees with lateral OA were associated with a reduced FO (P = 0.012), increased height of hip centre (HHC) (P = 0.003), more valgus neck-shaft angle (P = 0.042), and increased abductor angle (P = 0.031). Knees with medial OA were associated with a more varus neck-shaft angle (P = 0.043) and a decreased abductor angle (P = 0.003).

CONCLUSION

These data suggest anatomical variations at the hip and pelvis are associated with compartment-specific knee OA and may help to explain sex differences in patterns of knee OA.

Temporal trends in femoral diaphyseal torsional asymmetry among the Arikara associated with postural behavior

Average femoral torsion has been reported to differ among populations, and several studies have observed a relatively high prevalence of femoral anteversion asymmetry in Native Americans, especially females. This study investigates sexual dimorphism and temporal trends in femoral torsional asymmetry among the Arikara from the seventeenth to the early nineteenth century. To establish if there are population differences, femoral torsion was first measured using a direct method on a diverse comparative sample of Native Americans from the Southwest, Midwest, and Great Plains as well as American Whites and Blacks. To examine temporal trends among the Arikara, femoral torsion was examined using the orientation of the maximum bending rigidity at subtrochanteric in 154 females and 164 males from three temporal variants of the Arikara Coalescent tradition. There is significant sexual dimorphism in femoral torsional directional and absolute asymmetry among most Native American samples, but not among American Whites and Blacks. Among the Arikara there is significant sexual dimorphism in femoral torsional asymmetry in all three temporal variants, and asymmetry in femoral torsional asymmetry increased significantly from the protohistoric to the early historic period among females. The increased femoral torsional asymmetry is likely associated with a common side-sitting posture observed in historic photographs of Great Plains females. Historic Arikara females may have habitually sat in this compulsory position for extended periods while conducting domestic chores. The dramatic change from the protohistoric to historic period suggests a cultural change in sitting posture among females that was widespread across the Northern Plains.

How far above the true anatomic position can the acetabular cup be placed in total hip arthroplasty?

INTRODUCTION

There is controversy about which is more suitable for determining correct socket position in patients with severe bone deficiency of the acetabular roof because of developmental dysplasia of the hip (DDH): the anatomic centre of hip rotation or a high centre. 


METHOD

We evaluated the relationship, in 200 hips, between the centre of rotation and presence of the Trendelenburg sign to determine the upper limit of cup position from the standpoint of hip-abductor strength. 


RESULTS

Of the 200 hips, 20 (10%) showed a positive Trendelenburg sign. There were no statistically significant differences between parameters (the centre of rotation, femoral offset, abductor lever arm) regarding the presence of the Trendelenburg sign except for age at surgery. Patients with a positive Trendelenburg sign were significantly older (64.1 ± 9.4 years) than those with a negative Trendelenburg sign (58.8 ± 7.7 years) (P = 0.01).


CONCLUSIONS

Our findings indicate that a high centre of hip rotation of up to approximately 30 mm from the inter-teardrop line is a feasible option for patients with DDH from the standpoint of hip-abductor strength if stems are used that allow the restoration of femoral offset and the abductor lever arm.

Can squeaking with ceramic-on-ceramic hip articulations in total hip arthroplasty be avoided?

Squeaking is a recognized complication of total hip arthroplasty with ceramic on ceramic bearings but the etiology has not been well identified. We evaluated 183 hips in 148 patients who had undergone ceramic-on-ceramic noncemented total hip arthroplasties at one center between 1997-2007 by standardized telephone interviews and radiographic review. Audible squeaking was reported from 22 hips (12% of 183) of 19 patients. Prevalence of squeaking was associated with younger age; obesity; lateralized cup position; use of beta titanium alloy femoral components and shortened head length options; and higher reported activity level, greater pain, and decreased satisfaction at the time of the interview. Squeaking was described as having little personal significance by most patients. Squeaking might be preventable in part through medialization of the acetabular cup and avoidance of the use of shortened femoral necks.

Defining the normal acetabular vault in adult males and females using a novel three-dimensional model

The management and quantification of bone loss is a major challenge in primary and revision total hip replacement. Defining the normal three-dimensional (3D) anatomy of the acetabular vault could aid in assessing pathologic changes and in designing prosthetic joint components. We performed a quantitative assessment of normal 3D acetabular vault structure to define the shape and location of weight-bearing acetabular bone referred to as the vault. Images from 70 normal hip computed tomography images were used to define the 3D acetabular vault anatomy and develop a 3D model. Variation in vault shape was quantified by measuring the distance between every surface point on a subject's hemipelvis and the reference vault. Variation among different hip alignments was assessed using 19 scans from subjects with varus, valgus and dysplastic hip morphologies. The acetabular vault model had 96.6% (95% CI: 91.7-101.5), 97.8% (95% CI: 94.5-101.1) and 96.4% (95% CI: 98.7-94.1) of the surface points within 3 mm of normal male, normal female and abnormal hip specimens, respectively. Comparison of acetabular vault model fit between gender and hip types revealed that it was only significantly different between normal males and normal females (P = 0.0194) and between normal males and dysplastic females (P = 0.0377). A conserved 3D acetabular vault shape and location exists that can accommodate various hip morphologies. Defining a normal vault may increase the precision with which hip pathology can be identified and may also serve as a preoperative assessment tool for planning total hip arthroplasty.

Femoral loads during gait in a patient with massive skeletal reconstruction

BACKGROUND

Biological massive skeletal reconstructions in tumours adopt a long rehabilitation protocol aimed at minimising the fracture risk. To improve rehabilitation and surgical procedures it is important to fully understand the biomechanics of the reconstructed limb. The aim of the present study was to develop a subject-specific musculoskeletal model of a patient with a massive biological skeletal reconstruction, to investigate the loads acting on the femur during gait, once the rehabilitation protocol was completed.

METHODS

A personalised musculoskeletal model of the patient's lower limbs was built from a CT exam and registered with the kinematics recorded in a gait analysis session. Predicted activations for major muscles were compared to EMG signals to assess the model's predictive accuracy.

FINDINGS

Gait kinematics showed only minor discrepancies between the two legs and was compatible with normality data. External moments showed slightly higher differences and were almost always lower on the operated leg exhibiting a lower variability. In the beginning of the stance phase, the joint moments were, conversely, higher on the operated side and showed a higher variability. This pattern was reflected and amplified on the femoral forces where the differences became important: on the hip, a maximum difference of 1.6 BW was predicted. The variability of the forces seemed, generally, lower on the operated leg than on the contralateral one.

INTERPRETATION

Small asymmetries in kinematic patterns might be associated, in massive skeletal reconstruction, to significant difference in the skeletal loads (up to 1.6 BW for the hip joint reaction) during gait.

Level of subject-specific detail in musculoskeletal models affects hip moment arm length calculation during gait in pediatric subjects with increased femoral anteversion

Biomechanical parameters of gait such as muscle's moment arm length (MAL) and muscle-tendon length are known to be sensitive to anatomical variability. Nevertheless, most studies rely on rescaled generic models (RGMo) constructed from averaged data of cadaveric measurements in a healthy adult population. As an alternative, deformable generic models (DGMo) have been proposed. These models integrate a higher level of subject-specific detail by applying characteristic deformations to the musculoskeletal geometry. In contrast, musculoskeletal models based on magnetic resonance (MR) images (MRMo) reflect the involved subject's characteristics in every level of the model. This study investigated the effect of the varying levels of subject-specific detail in these three model types on the calculated hip MAL during gait in a pediatric population of seven cerebral palsy subjects presenting aberrant femoral geometry. Our results show large percentage differences in calculated MAL between RGMo and MRMo. Furthermore, the use of DGMo did not uniformly reduce inter-model differences in calculated MAL. The magnitude of these percentage differences stresses the need to take these effects into account when selecting the level of subject-specific detail one wants to integrate in musculoskeletal. Furthermore, the variability of these differences between subjects and between muscles makes it very difficult to a priori estimate their importance for a biomechanical analysis of a certain muscle in a given subject.

Aberrant pelvis and hip kinematics impair hip loading before and after total hip replacement

Musculoskeletal loading is an important factor affecting the development of osteoarthritis, bone remodelling and primary fixation of total hip replacement (THR). In this study, we analyzed the relation between muscular force, gait kinematics and kinetics and hip loading in 20 patients before and six weeks after THR. Hip contact forces were calculated from gait analysis data using musculoskeletal modelling, inverse dynamics and static optimization. We found aberrant pelvis and hip kinematics and kinetics before and six weeks after surgery, confirming previous findings in literature. Furthermore, we found a decrease in the total contact force and its vertical component. These changes result in a decrease of the associated inclination angles of the total hip contact force in the sagittal and transverse planes, changing the orientation towards more vertical implant loading after THR. These changes in hip loading were related to observed gait kinematics and kinetics. Most importantly, excessive pelvic obliquity and associated hip adduction related to impaired implant loading. We concluded, therefore, that physical therapy in the early post-operative phase should primarily focus on stretching of anterior and medial structures and strengthening of hip flexors and abductors to achieve normalization of the hip and pelvis kinematics and consequently normalize hip loading.

Subject-specific hip geometry and hip joint centre location affects calculated contact forces at the hip during gait

Hip loading affects the development of hip osteoarthritis, bone remodelling and osseointegration of implants. In this study, we analyzed the effect of subject-specific modelling of hip geometry and hip joint centre (HJC) location on the quantification of hip joint moments, muscle moments and hip contact forces during gait, using musculoskeletal modelling, inverse dynamic analysis and static optimization. For 10 subjects, hip joint moments, muscle moments and hip loading in terms of magnitude and orientation were quantified using three different model types, each including a different amount of subject-specific detail: (1) a generic scaled musculoskeletal model, (2) a generic scaled musculoskeletal model with subject-specific hip geometry (femoral anteversion, neck-length and neck-shaft angle) and (3) a generic scaled musculoskeletal model with subject-specific hip geometry including HJC location. Subject-specific geometry and HJC location were derived from CT. Significant differences were found between the three model types in HJC location, hip flexion-extension moment and inclination angle of the total contact force in the frontal plane. No model agreement was found between the three model types for the calculation of contact forces in terms of magnitude and orientations, and muscle moments. Therefore, we suggest that personalized models with individualized hip joint geometry and HJC location should be used for the quantification of hip loading. For biomechanical analyses aiming to understand modified hip joint loading, and planning hip surgery in patients with osteoarthritis, the amount of subject-specific detail, related to bone geometry and joint centre location in the musculoskeletal models used, needs to be considered.

The effect of elbow joint centre displacement on force generation and neural excitation

Joint centre displacement may occur following total elbow replacement due to aseptic loosening or surgical misalignment, and has been linked to implant failure. In this study, the effects of joint centre displacement were examined using a neuromusculoskeletal model of the elbow joint. Isometric contractions were simulated at a range of joint angles during elbow flexion and extension. Displacement of the joint centre affected the force-generating capacity about the joint, due to changes in both muscle lengths and moment arms. The magnitude and direction of the maximum joint reaction force were also altered, potentially contributing to aseptic loosening and compromising joint stability. The relationship between force generated and the level of neural excitation to the elbow flexor and extensor muscles was also affected, suggesting that altered neural control patterns could be required following joint centre displacement.

Can the acetabular position be derived from a pelvic frame of reference?

Acetabular center positioning has an effect on hip function. However, reported clinical and plain radiographic methods are inaccurate and unreliable for ascertaining acetabular implant location. In an exploratory study we asked whether the normal acetabular position can be derived from simple radiographically measurable pelvic dimensions. We analyzed computed tomographic scans of 37 normal hips using a pelvic frame of reference centered on the ipsilateral anterior-superior iliac spine. We defined the x-, y-, and z-coordinates of the hip center (C(x),C(y),C(z)) as a percentage of the corresponding pelvic dimensions (D(x),D(y),D(z)). C(x)/D(x) averaged 9%, C(y)/D(y) 34%, and C(z)/D(z) 37%. These ratios had narrow distributions with small confidence intervals. Interobserver agreement tests showed a mean intraclass correlation coefficient of 0.95. We observed gender differences in the ratios of as much as 4%, which correspond to differences of as much as 9 mm in the hip center position. The ratios provide a simple and reliable way of deriving the normal position of the hip center from the pelvic dimensions alone. This gives the surgeon a simple way of planning where the hip center should be and may be particularly helpful in revision hip arthroplasty or in cases involving extensive osteophytes, dysplasia, or protrusio.

The influence of the centre of rotation on implant survival using a modular stem hip prosthesis

The restoration of the hip centre of rotation in an anatomical position is considered to be relevant for total hip prosthesis survival. When the cup is implanted with a high centre of rotation, the lever arm of the abductor muscles is decreased, causing higher joint-reaction forces. Modular stems with varying lengths and geometries can be used to balance soft tissues, and ceramic bearing surfaces can be used to reduce the wear rate. Forty-four hip replacements performed with a high hip centre of rotation were matched with 44 performed with an anatomical centre of rotation. In all cases the preoperative diagnosis was dysplasia of the hip (DDH) and cementless modular neck prostheses with ceramic bearing surfaces were used. At nine years follow-up the mean Harris hip and WOMAC scores were not statistically different. All stems and cups were stable; the femoral offset was no different between the two groups (p = 0.4) as leg-length discrepancy (p = 0.25).

OpenSim: open-source software to create and analyze dynamic simulations of movement

Dynamic simulations of movement allow one to study neuromuscular coordination, analyze athletic performance, and estimate internal loading of the musculoskeletal system. Simulations can also be used to identify the sources of pathological movement and establish a scientific basis for treatment planning. We have developed a freely available, open-source software system (OpenSim) that lets users develop models of musculoskeletal structures and create dynamic simulations of a wide variety of movements. We are using this system to simulate the dynamics of individuals with pathological gait and to explore the biomechanical effects of treatments. OpenSim provides a platform on which the biomechanics community can build a library of simulations that can be exchanged, tested, analyzed, and improved through a multi-institutional collaboration. Developing software that enables a concerted effort from many investigators poses technical and sociological challenges. Meeting those challenges will accelerate the discovery of principles that govern movement control and improve treatments for individuals with movement pathologies.

Image-based musculoskeletal modeling: Applications, advances, and future opportunities

Computer models of the musculoskeletal system are broadly used to study the mechanisms of musculoskeletal disorders and to simulate surgical treatments. Musculoskeletal models have historically been created based on data derived in anatomical and biomechanical studies of cadaveric specimens. MRI offers an abundance of novel methods for acquisition of data from living subjects and is revolutionizing the field of musculoskeletal modeling. The need to create accurate, individualized models of the musculoskeletal system is driving advances in MRI techniques including static imaging, dynamic imaging, diffusion imaging, body imaging, pulse-sequence design, and coil design. These techniques apply to imaging musculoskeletal anatomy, muscle architecture, joint motions, muscle moment arms, and muscle tissue deformations. Further advancements in image-based musculoskeletal modeling will expand the accuracy and utility of models used to study musculoskeletal and neuromuscular impairments.

System for intraoperative evaluation of soft-tissue-generated forces during total hip arthroplasty by measurement of the pressure distribution in artificial joints

A system for evaluating the soft-tissue-generated forces at the hip joint was developed. The system enabled measurement of contact pressure distribution at hip joint surfaces, as well as evaluation of the artificial hip joint condition during total hip arthroplasty (THA). First, a pressure sensor module that forms part of the artificial joint was constructed. Eight small pressure sensors were installed in the spherical head component of the ball-and-socket joint. Next, software for recording and visualizing the detected pressures at 1-millisecond intervals was developed. The pressure distribution was displayed in real time via 3D computer graphics on a monitor. The system enabled intuitive recognition of the direction of soft-tissue-generated forces and pressure distribution in three dimensions. Accuracy tests were conducted using a high-accuracy 6-degree-of-freedom positioning device and digital force gauge. The error between the applied loads and measured forces was 3.42 +/- 3.26 N (mean +/- standard deviation) for each coordinate in 10 trials involving load application from 10 different directions. Next, a clinical evaluation was conducted during THA. The relative positions of the cup and stem component were measured using a surgical navigation system simultaneously with the pressure measurement. The system allowed real-time acquisition of information regarding the artificial hip joint, as well as comparison of the differences in the hip condition when several types of neck were used. Further improvements to the calibration method should enable more accurate measurements. We believe this system will be a useful tool for selecting an appropriate implant that fits a patient's hip joint or for estimating the risk of complications following surgery.

Four-dimensional model of the lower extremity after total hip arthroplasty

We have developed a four-dimensional (4D) model of the lower extremities after total hip arthroplasty in patients. The model can aid in preventing complications such as dislocation and wearing of the sliding surface. The skeletal structure and implant alignment were obtained from CT data. We applied registration method using CAD data to estimate accurate implant alignment from scattered CT data. The reconstructed three-dimensional (3D) skeletal model was combined with motion capture data that were acquired by an optical tracking system. We displayed the patient's skeletal movement and analyzed several parameters that relate to complications. The patient's skeletal model was superimposed onto video footage that was taken by a synchronized and calibrated digital video camera. For validation of the measurement error in this system, we used open MRI to evaluate the relative movement between skin markers and bones. This system visually represents not only the 3D anatomical structure, but also 4D dynamic functions that include the time sequential transitions of components and their positions. The open MRI results indicated that the average error in hip angle was within 5 degrees for each static posture. This system enables clinicians to analyze patient's motions on the basis of individual differences. We found that our system was an effective tool in providing precise guidance of daily postoperative motions that was individualized for each patient. This system will be applicable for surgical planning, assessment of postoperative activities, and the development of new surgical techniques, materials, and prosthetic designs.

Greater trochanteric transfer for the treatment of coxa brevis

We reviewed the outcomes of 28 patients for whom we did greater trochanteric transfer. Twenty-two (79%) patients treated for coxa brevis by greater trochanteric transfer had good relief of pain and limp. Four (1%) outcomes were fair and two (0.7%) were poor. Our outcomes compared favorably with those already reported in the literature. Patients who had avascular necrosis caused by Perthes disease tended to have a better outcome than those caused as a complication of treatment for hip dysplasia. A computer model based on radiographic measurements was useful in planning the placement of the transfer for biomechanical efficiency. The surgical technique and osteotomy planning is described. Our goal is to review (1) the effectiveness of our surgical procedure, (2) whether deformity etiology influences outcome, (3) the best way to achieve optimal biomechanical efficiency, and (4) how software assisted planning can assist in surgical planning.

Dynamic coordinate data for describing muscle–tendon paths: a mathematical approach

When modelling the musculoskeletal system over a range of joint angles the use of fixed points to describe muscle-tendon paths has inherent limitations. These result in fewer deflection points and the use of effective insertions to accommodate both relative marker movement and avoid muscle paths contacting bony structures. Model performance is dependent on the joint angle relative to the anatomical position where the muscle-tendon paths were defined. The present study proposes a scheme for the implementation of dynamic coordinates for describing muscle-tendon paths. For each muscle-tendon element a plane is defined in which the muscle-tendon complex acts when crossing a given joint. The muscle-tendon plane is dependent on 3D segment orientations and describes one degree of freedom, while the remaining two degrees of freedom are described by polar coordinates and locate the dynamic point in the muscle-tendon plane. The dynamic approach is implemented on four muscles of the lower limb in modelled and simulated joint movements and offers a significant improvement on previous approaches based on fixed deflection points. The scheme accommodates compound 3D rotations about joint axes, is not computationally difficult or require large data sets, and does not impose limitations on the number of points that may be defined along a muscle-tendon path.

[Change of offset after implantation of hip alloarthroplasties]

The purpose of the present study was to evaluate the change of the offset after implantation of hip alloarthroplasties. In an experimental study the X-ray templates of 90 different implants (594 different sizes) were digitized (cup and stem) and virtually implanted in the hip joint of 50 consecutive patients who were on the waiting list for hip replacement. Implantation was performed on AP X-rays paying heed to the fit of the shaft and adequate leg length. In total 4500 implantations were performed. After virtual implantation the change of the offset was calculated. There was a wide variance in the ability to fit the implants to the individual patient. For some patients it seems to be extremely difficult to adapt the implant without changing the offset. Some implant designs were only suitable for 2 patients while other designs were suitable for 40 patients (mean: 17). The average change of the offset was 0.27 cm. Considering only the cases in which a change occurs this value increases up to 0.56 cm.For the individual patient the average number of implants that were suitable was 30 (range: 1-67) of a total of 90 designs. Here the average change of the offset was 0.4 cm (-1.04 -- +1.54 cm). Considering only the cases in which a change occurs this value increases up to 0.64 cm. In the majority of the cases the presently available implants do not allow anatomic reconstruction of the individual offset.

New aspects and approaches in pre-operative planning of hip reconstruction: a computer simulation

BACKGROUND

All computer-aided surgery technologies assume that the surgeon knows the best position for the implant components. However, there is indirect evidence that simple anatomical information may not be sufficient for the surgeon to decide size and position of the implant in a repeatable manner.

METHOD

In the present study we estimated the variability in choosing the size and position of the components of a cementless total hip replacement (THR), using template-on-radiograph as well as computed tomography (CT)-based computer-aided planning. In addition, using a computer model, we assessed the sensitivity to such variability of implant fitting, location of the joint centre, skeletal range of motion, and resting length of major hip muscles. Using templates, surgeons selected the size with variability up to 2.5 mm for the stem and up to 4 mm for the socket. A similar variability was also observed when the CT-based planning program was used.

RESULTS

No major differences were found between surgeons. The standard deviation over repeated planning sessions of the selected position for each component, using the CT-based planning software, was found to be 3.9 mm and 8.9 degrees .

CONCLUSION

On the basis of the computer simulation, this variability did not affect the selected biomechanical parameters in a drastic way, although some differences were observed, especially in the lever arm of the hip muscles.

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

Aseptic loosening of artificial hip joints is believed to be influenced by the design and orientation of the implant. It is hypothesised that variations in implant anteversion and offset lead to changes in the loading of the proximal femur, causing critical conditions to both the bone and cement. The goal of this study was therefore to analyse the role of these parameters on loading, bone strains and cement stresses in total hip arthroplasty (THA). A validated musculo-skeletal model was used for the analysis of muscle and joint contact forces during walking and stair climbing. Two different anteversion angles (4 degrees vs. 24 degrees ) and prostheses offsets (standard vs. long) were analysed. The loads for each case were applied to a cemented THA finite element model. Generally, stair climbing caused higher bone strains and cement stresses (max. +25%) than walking. Variations in anteversion and offset caused changes in the loading environment, bone strain distribution and cement stresses. Compared to the standard THA configuration, cement stresses were raised by increasing anteversion (max. +52%), offset (max. +5%) and their combination (max. +67%). Femoral anteversion, offset and their combination may therefore lead to an increased risk of implant loosening. Analyses of implant survival should consider this as a limiting factor in THA longevity. In clinical practice, implant orientation, especially in regard to pre- and post-operative anteversion, should be considered to be more critical.

A new parametric approach for modeling hip forces during gait

An analytical parametric model was developed to estimate the natural biological variations in muscle forces and their effect on the hip forces subject only to physiological constraints and not predefined optimization criterion. Force predictions are based on the joint kinematics and kinetics of each subject, a previously published muscle model, and physiological constraints on the muscle force distributions. The model was used to determine the hip contact forces throughout the stance phase of gait of a subject with a total hip replacement (THR). The parametrically modeled peak hip force without antagonistic muscle activity varied from 2.7 to 3.2 Body Weights (mean 2.9 Body Weights), which agreed well with published in vivo measurements from instrumented THRs in other subjects. For every 10% increase in antagonistic activity, the mean peak hip force increased by 0.2 Body Weights. The parametric model allows one to examine the effect of specific muscle weaknesses or increased antagonistic muscle activity on the hip forces. The model also provides a tool for studying the effect of gait adaptations on hip forces, as predictions are based on each individual's gait data. Differences in peak forces between subjects can then be evaluated relative to the uncertainty in not knowing the precise muscle force distributions.

A Mathematical Formulation for 3D Quasi-Static Multibody Models of Diarthrodial Joints

This study describes a general set of equations for quasi-static analysis of three-dimensional multibody systems, with a particular emphasis on modeling of diarthrodial joints. The model includes articular contact, muscle forces, tendons and tendon pulleys, ligaments, and the wrapping of soft tissue structures around bone and cartilage surfaces. The general set of equations governing this problem are derived using a consistent notation for all types of links, which can be converted conveniently into efficient computer codes. The computational efficiency of the model is enhanced by the use of analytical Jacobians, particularly in the analysis of articular surface contact and wrapping of soft tissue structures around bone and cartilage surfaces. The usefulness of the multibody model is demonstrated by modeling the patellofemoral joint of six cadaver knees, using cadaver-specific data for the articular surface and bone geometries, as well as tendon and ligament insertions and muscle lines of actions. Good accuracy was observed when comparing the model patellar kinematic predictions to experimental data (mean +/- stand. dev. error in translation: 0.63 +/- 1.19 mm, 0.10 +/- 0.71 mm, -0.29 +/- 0.84 mm along medial, proximal, and anterior directions, respectively; in rotation: -1.41 +/- 1.71 degrees, 0.27 +/- 2.38 degrees, -1.13 +/- 1.83 degrees in flexion, tilt and rotation, respectively). The accuracy which can be achieved with this type of model, and the computational efficiency of the algorithm employed in this study may serve in many applications such as computer-aided surgical planning, and real-time computer-assisted surgery in the operating room.

Size and direction of hip joint forces associated with various positions of the acetabulum

When total hip replacement is performed, the position of the acetabular component may affect wear and component survival time. We considered the questions: In what way does displacement of the hip joint center alter (1) the magnitude and (2) the direction of the resultant force? Biomechanical tests were carried out on a human multibody model. After displacement of the hip joint center, the resultant forces were calculated for the single leg stance. With the flexed single leg stance, maximum hip joint forces were observed with lateral, cranial, posterior displacement. The peak forces were affected by the modeling of a gluteus maximus wrapping point at the ischial tuberosity and were overestimated when this was removed. With the straight single leg stance, posterior displacement decreases the total load on the hip joint because of the increased leverage of the rectus femoris. With regard to the direction of the resultant force, medial displacement increases the angles in both planes, cranial displacement increases it in the sagittal plane (cranial, posterior-caudal, anterior), and anterior displacement decreases the angle in the sagittal plane and increases it in the frontal plane (medial, cranial-lateral, caudal). The direction of the force is relatively insensitive to displacement of the hip joint center. The results presented here indicate a marked increase in the force after lateral, cranial, posterior displacement of the center in the flexed single leg stance. To avoid extreme joint loading and to reduce the wear after total hip arthroplasty, the cranial and posterior regions of the acetabulum should be fully reconstructed. A high hip joint center has an adverse effect on the magnitude of the force, although the directions are hardly affected by it.

An experimental method for the application of lateral muscle loading and its effect on femoral strain distributions

Experimental models that have been used to evaluate hip loading and the effect of hip implants on bone often use only a head load and abductor load. Anatomic considerations and in vivo measurements have lead several investigators to suggest that these models are inaccurate because they do not incorporate the loads imposed by additional muscles. The aim of this study was to evaluate the strains in the proximal and mid diaphysis of the femur for five hip loading models, one with a head load and abductor load only and four which incorporated lateral muscle loads as well. Head load to body weight load ratios were used to evaluate the physiologic accuracy of these models and strains were compared to determine the extent of strain changes as a function of model complexity. All models which incorporated additional lateral muscle loads more accurately simulated head load to body-weight load ratios than the simple abductor-only model. The model which incorporated a coupled vastus lateralis and iliotibial band load in addition to the abductor load provided the simplest configuration with a reasonable body-weight to head-load ratio.

Comparison of human, primate, and canine femora: implications for biomaterials testing in total hip replacement

The canine model remains an animal of choice for determining the efficacy and safety of various materials and designs used in human total hip replacement (THR). The primate also is used in orthopedic-related research for studying limb anatomy, gait, and age-related bone loss. In order to better understand the appropriateness of these animal models for human THR, external morphologies of thirty-three adult Caucasian human, sixteen adult chimpanzee, and forty-two adult greyhound femora were compared using osteometric methods. Measured parameters included anteversion angle, cervico-diaphyseal angle, femoral head offset in the frontal plane, and anterior bow profiles along the femoral diaphysis. Although some of the measured parameters were approximately similar between species (e.g., mean cervico-diaphyseal angle of humans and chimpanzees), the majority demonstrated morphologic differences that may be biomechanically significant for interpreting stress transfer across the hip (e.g., mean anteversion angle and mean normalized femoral head offset between species). Additionally, age-related changes in proximal femoral morphology and gait pattern, as well as species-related differences in local muscle and inertial forces, may result in notably different loading conditions across the hip joint of each species. Therefore, discretion must be exercised when evaluating canine or primate THR materials and designs for potential use in the human hip.

Internal rotation gait: a compensatory mechanism to restore abduction capacity decreased by bone deformity

Children with excessive femoral anteversion frequently walk with abnormal internal rotation of the hip. The authors hypothesized that excessive anteversion decreases the abduction moment arm of the gluteus medius and that this moment arm is restored with internal rotation; hence internal rotation may be a compensatory mechanism to preserve abduction capacity. To test this hypothesis a three-dimensional computer model of an adult lower limb was developed to determine how changes in femoral anteversion angle, neck-shaft angle, and hip internal rotation angle affect the abduction moment arm of the gluteus medius. Analysis of the model revealed that anteversion and valgus deformities of the femur can decrease the abduction moment arm of the gluteus medius substantially. In particular, increasing the anteversion angle of the model by 30 to 40 degrees caused a 40 to 50% decrease in the abduction moment arm of the gluteus medius - enough to impair walking. Internal rotation of the hip by 30 degrees restored the abduction moment arm of the gluteus medius to within 5% of the moment arm of the model in its normal, undeformed state. These results support the authors' hypothesis and are consistent with the theory that internal rotation may be a compensatory mechanism adopted by children with femoral deformities to achieve the abduction moment arm needed for walking.

How superior placement of the joint center in hip arthroplasty affects the abductor muscles

This study examines the effects of a superiorly placed hip center on the strength of the abductor muscles. A 3-dimensional computer model of the hip and the surrounding musculature was used to calculate the moment arms, forces, and moments generated when the hip abductor muscles are maximally activated. A representation of a hip prosthesis was implanted into the computer model with altered hip center positions and a range of prosthetic neck lengths. Analysis of these simulated hip replacements demonstrated that superolateral placement of the hip center (2 cm superior and 2 cm lateral) decreases the moment arms of the hip abductor muscles by an average of 28%. This decrease in moment arm cannot be restored by increasing prosthetic neck length, resulting in an unrecoverable loss of abduction strength with superolateral displacement. By contrast, a 2-cm superior displacement of the hip center changes the moment arms and force generating capacities of the abductors by less than 10% if prosthetic neck length is increased to compensate for decreased muscle length. The results of this study suggest that superior positioning of the hip center, without lateral placement, does not have major, adverse effects on abduction moment arms or force generating capacities when the neck length is appropriately increased.

Trochanteric transfer in total hip replacement: effects on the moment arms and force-generating capacities of the hip abductors

A three-dimensional computer model of the pelvis, femur, gluteus medius, and gluteus minimus was used to evaluate the changes in muscle moment arms and force-generating capacities caused by alterations in the location of the greater trochanter. In the first part of this study, the hip center and all other aspects of joint geometry remained unaltered, while we examined changes in abduction moment arms that resulted from transfer of the trochanteric fragment to a wide variety of positions on the femur. The largest increase in average abduction moment arm was 11% (0.5 cm), which occurred with an anterolateral transfer. Most transfers resulted in moment arm changes of less than 5%. In the second part of this study, the hip center was displaced 2 cm superiorly, and the effects of a distal trochanteric transfer on the moment arms and force-generating capacities of the abductors were analyzed. The superior displacement caused a 13% decrease in the moment arm of the abductors and a 43% decrease in their force-generating capacity. The moment arm was not restored by distal transfer of the greater trochanter; however, distal transfer had the major advantage of restoring muscle lengths and force-generating capacities. These results suggest that trochanteric transfer should be considered primarily as a means to restore muscle length because it has limited potential to increase the moment arms of the two primary hip abductors.

A graphics-based software system to develop and analyze models of musculoskeletal structures

We have created a graphics-based software system that enables users to develop and analyze musculoskeletal models without programming. To define a model using this system one specifies the surfaces of the bones, the kinematics of the joints and the lines of action and force-generating parameters of the muscles. Once a model is defined, the function of each muscle can be analyzed by computing its length, moment arms, force and joint moments. The software has been implemented on a computer graphics workstation so that users can view the model from any perspective and graphically manipulate the joint kinematics and musculoskeletal geometry. Models can also be animated to visualize the results of motion analysis experiments. Since the software can be used to study models of many different musculoskeletal structures, it can enhance the productivity of investigators working on diverse problems in biomechanics.