Force- and moment-generating capacity of lower-extremity muscles before and after tendon lengthening.

A computer model of the human lower extremity was developed to study how surgical lengthening of tendon affects the force- and moment-generating capacity of the muscles. This model computes the maximum isometric force and the resulting joint moments that each muscle-tendon complex can develop at any body position. Tendon lengthenings were simulated by increasing the tendon length of each muscle-tendon complex and computing the change in the maximum isometric muscle force and joint moments at a specific body position. These simulations showed that the forces and moments developed by the ankle plantarflexors are extremely sensitive to changes in tendon length. For example, at a body position corresponding to the midstance phase of gait, the maximum isometric moment generated by soleus decreased 30% with a 1-cm increase in tendon length, and 85% with a 2-cm increase in tendon length. In contrast, 1- and 2-cm increases in iliopsoas tendon length decreased its hip flexion moment by only 4% and 9%, respectively. This article quantifies the sensitivity of muscle force and joint moments to changes in tendon length for the most commonly lengthened lower-extremity tendons. These results indicate how much each of these tendons should be lengthened to achieve an incremental decrease in muscle force or joint moment.     

Change in the CCD angle and the femoral anteversion angle by hip prosthesis implantation]

PURPOSE: The aim of the present study was to measure and interpret the change of the collodiaphyseal (CCD) angle and femoral anteversion after total hip replacement. METHODS: We prospectively examined 52 patients with coxarthrosis, who were treated by total hip replacement. Preoperatively and postoperatively we used a standard X-ray ap view to measure the CCD angle and computerized tomography to determine the femoral anteversion. These projected angles were converted into the real angles by using the method of K?nig and Schult. RESULTS: On average the preoperative real CCD angle was 128 degrees (+/- 8.9 degrees) and the postoperative angle 145 degrees (+/- 4.8 degrees), preoperatively the real femoral anteversion angle was 14.1 degrees (+/- 6.9 degrees) and postoperatively 10.8 degrees (+/- 6.2 degrees). CONCLUSIONS: The effect of the postoperative valgisation can be explained by the given CCD angle of the used femoral stem prosthesis of 140 degrees (cemented Weller II stem) and 145 degrees (cementless CLS classic stem). It is possible that due to the valgisation and the decreased offset of the femoral stem prosthesis compared to the preoperative conditions the gluteal muscles are insufficient and overloaded. It is also known that the change of the femoral anteversion from the physiological conditions causes an increase of the torsional moment. The resulting increased interface load could possibly be a reason for loosening of the femoral stem. The conclusion can be drawn that the CCD angle and the femoral anteversion should more carefully be considered by the surgeon in total hip replacement. This could be reached by an exact implantation technique and the choice of the appropriate stem prosthesis with different CCD angles.     

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.     

Femoral Neck Angles in Osteoarthritis of the Hip

Femoral neck angles were determined in 44 patients with unilateral or bilateral idiopathic osteoarthritis of the hip. The mean anteversion angle was 20° ± 9° and the neck-shaft angle 131° ± 7°. In comparison with a normal material the anteversion was significantly larger in the patients (P < 0.001), but no difference was found in the neck-shaft angle. A positive correlation between the size of the anteversion and the severity of the disease was observed.

It is concluded that increased anteversion of the femoral neck may contribute to the development of osteoarthritis of the hip.     

Is Combined Anteversion Equally Affected by Acetabular Cup and Femoral Stem Anteversion?

BackgroundTo create a safe zone, an understanding of the combined femoral and acetabular mating during hip motion is required. We investigated the position of the femoral head inside the acetabular liner during simulated hip motion. We hypothesized that cup and stem anteversions do not equally affect hip motion and combined hip anteversion.MethodsHip implant motion was simulated in standing, sitting, sit-to-stand, bending forward, squatting, and pivoting positions using the MATLAB software. A line passing through the center of the stem neck and the center of the prosthetic head exits at the polar axis (PA) of the prosthetic head. When the prosthetic head and liner are parallel, the PA faces the center of the liner (PA position = 0, 0). By simulating hip motion in 1-degree increments, the maximum distance of the PA from the liner center and the direction of its movement were measured (polar coordination system).ResultsThe effect of modifying cup and stem anteversion on the direction and distance of the PA’s change inside the acetabular liner was different. Stem anteversion influenced the PA position inside the liner more than cup anteversion during sitting, sit-to-stand, squatting, and bending forward (P = .0001). This effect was evident even when comparing stems with different neck angles (P = .0001).ConclusionCup anteversion, stem anteversion, and stem neck-shaft angle affected the PA position inside the liner and combined anteversion in different ways. Thus, focusing on cup orientation alone when assessing hip motion during different daily activities is inadequate.     

Corrosion of a hip stem with a modular neck taper junction: a retrieval study of 16 cases

Since the early 1990s, there has been a dramatic increase in modular total hip designs ranging from a stem with a proximal taper and modular head, to a distal stem, double taper proximal neck, and modular head. Clinical advantages of the modular neck include intraoperative adjustment of leg length via the neck-head taper and femoral anteversion via the neck-stem taper. Sixteen cases of a double tapered cone, Margron hip prosthesis, were presented for retrieval analysis. Macroscopic inspection, corrosion testing, light microscopy, and scanning electron microscopy were conducted to elucidate mechanisms of failure. In this regard, 6 neck components showed significant fretting, and crevice corrosion of the neck-stem taper with an average implantation time of 39 months compared with the remaining retrievals, which showed no corrosion with and average time in situ of 2.7 months. This retrieval study demonstrates that even with a modern taper design and corrosion-resistant materials, increased modularity can lead to fretting and crevice corrosion, metal ion generation, and particulate debris that may contribute to periprosthetic osteolysis and loosening.     

Moment arm and force-generating capacity of the extensor carpi ulnaris after transfer to the extensor carpi radialis brevis.

Tendon transfers to the extensor carpi radialis brevis (ECRB) are often performed to augment wrist extension. This study was conducted to analyze how transfer of the extensor carpi ulnaris (ECU) to the ECRB affects the moment arms, force-generating capacity, and moment-generating capacity of the ECU over a range of wrist flexion-extension. A graphics-based computer model was developed from anatomic measurements of the muscle-tendon paths before and after transfer. This model calculates the lengths and moment arms of the muscles over a range of wrist flexion-extension and represents the muscles' force-generating characteristics from previous measurements of their physiologic cross-sectional areas, fiber lengths, and pennation angles. Analysis of the computer model revealed that the maximum isometric extension moment of the ECU at the neutral wrist position increased from 0.50 N-m to 1.72 N-m after transfer to the ECRB. The deviation moment shifted from 2.72 N-m ulnar deviation to 1.42 N-m radial deviation. The extension moment generated by the ECU varied more with wrist flexion angle after transfer due to its broadened operating range on the muscle force-length relationship. The simulations highlight the need for proper intraoperative tensioning of the ECU to maximize the force-generating potential of the transferred muscle over the functional range of motion.     

The optimal combined anteversion pattern to achieve a favorable impingement-free angle in total hip arthroplasty

BackgroundThere have been no studies on the differences in impingement-free angle that result from different combined anteversion (CA) patterns. The aim of this study was to find the optimal CA pattern for achieving a favorable impingement-free angle, including bony and prosthetic impingement, in total hip arthroplasty.MethodsWe evaluated 100 patients with no hip arthritis. We investigated the impingement-free angle (flexion, internal rotation with 90° flexion, extension, and external rotation) after changing the stem and cup anteversions to satisfy several CA patterns [cup anteversion + stem anteversion = 30°, 40°, 50°, and 60°; cup anteversion + 0.7 × stem anteversion = 37.3° (:Widmer's theory); and cup anteversion + 0.77 × stem anteversion = 43.3° (:Yoshimine's theory)] using 3-dimensional templating software.ResultsThe impingement-free angle changed dramatically among the various CA patterns. The optimal CA was changed by various stem anteversion. Only CA: Widmer with stem anteversion of 20° satisfied daily-life range of motion (ROM) requirements (flexion ≥130°, internal rotation with 90° flexion ≥ 45°, extension ≥ 40°, external rotation ≥ 40°).ConclusionGood impingement-free angle cannot be obtained with single fixed CA. Different CA patterns should be used, depending on the differences in the stem anteversion. A CA of 30° with 0° ≤ stem anteversion ≤10°; a CA:Widmer with 20° of stem anteversion; a CA of 40° or Widmer with 30° of stem anteversion. When stem anteversion is ≥40°, CA should be decided by each patient's state. Among them, a stem anteversion of 20° with cup anteversion of 23.3° was found to be the best CA pattern.     

Analysis of optimal range of socket orientations in total hip arthroplasty with use of computer-aided design simulation

A three-dimensional computer-aided design model of a total hip replacement was used to study the effects of anteversion and abduction of the acetabular component and anteversion and varus-valgus an-gulation of the femoral component on the range of hip flexion and extension that could be obtained without component impingement. Impingement of the component was defined as impingement between the neck of the femoral component and the edge of the acetabular component. To achieve an angle of hip flexion greater than 90° and an extension angle greater than 30° without component impingement, the optimal angulations were found to be between 1 and 30° of anteversion and 30 and 50° of abduction of the acetabular component, as well as 10° of anteversion of the femoral component. When the valgus angulation of the femoral component was reduced from 7 to 0°, the allowable range of flexion without impingement increased under the. same conditions of acetabular-component orientation and femoral-component anteversion. Significant inverse correlations were found between the anteversion angle of the acetabular component and both the lumbar lordosis angle and the sacrohorizontal angle.     

Changes of biomechanical parameters in dysplasia of the hip by total hip replacement

AIM: Dysplastic hips or dislocated hips lead to loss of muscle strength claudication, and finally to coxarthrosis. This study analytically compares the hip joint forces in normal, dysplastic hips and hips after implantation of a hip prosthesis for several positions. METHOD: The results of 173 total hip replacements in 153 cases of either severe congenital dysplasia or dislocation were analyzed. A 2 dimensional mathematical model of the hip was developed to evaluate the effects of surgically achievable mechanical alterations such as acetabular placement, femoral shaft-prosthetic neck angle, and neck length of the femoral prosthesis. RESULTS: 63 % of the hip prosthesis were implanted on the right, 72 % on the left. Before implantation, the femoral shaft-prosthetic neck angle in mean was 146 degrees +/- 10 degrees, after implantation it was 135 degrees. The hip centre was displaced on in average 10 mm distally. Postoperatively hip joint force was increased 21 %. CONCLUSION: Minimum joint contact forces occurred when the femoral shaft prosthetic neck angles were small. The loads on the hip were lowered significantly by placing the centre of the acetabulum as far inferiorly and medially as possible. Another important finding is that displacement of the hip centre distally has a great effect on muscle performance and hip joint force.     

全髋关节置换术中股骨横向偏心距重建策略的研究进展

作为全髋关节置换术(total hip arthroplasty,THA)的一重项要指标,偏心距的重建及重建程度对疾病预后、患者生活质量的提高起着至关重要的作用。股骨偏心距的重建与股骨距长度保留、假体头颈长度、颈干角大小、下肢是否等长密切相关。重建策略包括术前细致规范的模板测量,股骨距预测及长度保留,术中选用近似解剖颈干角的假体,假体头颈的长度调整等。本文就偏心距的重建策略思想及偏心距对髋关节功能、假体磨损、术后疼痛、跛行、双下肢不等长等并发症的影响及相关研究进展做一综述。     

全髋关节表面置换术治疗成人髋关节发育不良中假体的安放技术和临床疗效分析

目的探讨全髋关节表面置换术治疗髋关节发育不良（DDH）术中假体安放位置的控制和临床效果。方法2005年1月至2007年9月，对42例45髋因DDH继发骨关节炎的患者行全髋关节表面置换术。其中女39例，男3例，平均年龄46．3岁，单侧39例39髋，双侧3例6髋。按Crowe分型，Ⅰ型17例18髋，Ⅱ型17例19髋，Ⅲ型6例6髋，Ⅳ型2例2髋。术前通过CT三维重建测量股骨颈干角、股骨颈前倾角。入路采用改良后外侧Gibson入路，根据术前测量结果，对于颈干角小于135°，手术时适当增加至135°，大于135°则维持原有角度；DDH患者股骨前倾角均有增加，术中应适当减少，并减少髋臼前倾角和外展角。采用X线检查和Harris功能评分评估术后疗效。结果术前股骨颈干角平均134．1°，术后平均138.2°，其中术前颈干角小于135°的34髋，平均131．7°，术后平均137°；术前颈干角大于135°的12髋，平均140.9°，术后平均141．5°。股骨前倾角术前平均34．5°。术后X线显示所有髋臼均为真臼重建，髋臼外展角平均42°。双侧肢体长度差别术前平均2.1cm，术后平均0．5cm。平均随访14．6个月。Harris功能评分术前平均43．6分，最后一次随访功能评分为平均88．4分。随访期内无股骨颈骨折和假体松动等并发症发生。结论对年青DDH并骨关节炎患者采用髋关节表面置换术的近期效果满意，术中根据个体情况调整假体安放位置有助于提高临床效果。     

Fifteen year-experience with the angle bore socket in patients with an increased risk of dislocation following total hip replacement

The angle bore socket was designed by surgeons at the Charnley Institute in Wrightington and introduced clinically in 1983. The socket was designed to provide increased coverage to the prosthetic femoral head during flexion, adduction and internal rotation, in an effort to decrease the rate of dislocation. It achieves this by using a chamfer, which is angled at 45° to the bore of the socket and is discontinuous anteriorly, preventing impingement between the socket and prosthetic femoral neck during flexion, adduction and internal rotation. We report the 15-year experience of the senior author with the angle bore socket in patients with an increased dislocation risk, secondary to increased age with general poor mobility and muscle function, decreased cognitive ability, recurrent dislocation and decreased muscle function secondary to co-morbid illness. Between 1990 and 2005, 54 patients underwent a total hip replacement using an angle bore socket. Of these patients, only one suffered a recurrent episode of dislocation post-operatively, requiring further surgery. The dislocation rate of 1.85% seen in this study suggests that this prosthesis can decrease the incidence of dislocation in vulnerable patients. In these patients, with a low activity level and/or short life expectancy, the benefit of the increased stability provided by the angle bore socket outweighs the limitations imposed by increased constraint.     

Bony impingement affects range of motion after total hip arthroplasty: A subject‐specific approach

Hip range of motion after total hip arthroplasty has been shown to be dependent on prosthetic design and component placement. We hypothesized that bony anatomy would significantly affect range of motion. Computer models of a current generation hip arthroplasty design were virtually implanted in a model of pelvis and femur in various orientations ranging from 35° to 55° cup abduction, 0° to 30° cup anteversion, and 0° to 30° femoral anteversion. Four head sizes ranging from 22.2 to 32 mm and two neck sizes ranging from 10‐mm and 12‐mm diameter were tested. Range of motion was recorded as maximum flexion–extension, abduction–adduction, and axial rotation of the femur before any contact between prosthetic components or bone was detected. Bony impingement preceded component impingement in about 44% of all conditions tested, ranging from 66% in adduction to 22% in extension. Range of motion increased as head size increased. However, increasing head size also increased the propensity for bony impingement, which tended to reduce the beneficial effect of increased head size on range of motion. Reducing neck diameter had a greater effect on prosthetic impingement (mean, 3.5° increase in range of motion) compared to bone impingement (mean, 1.9°). This model allowed for a clinically relevant assessment of range of motion after total hip arthroplasty and may also be used with patient‐specific geometry [such as that obtained from preoperative computed tomography (CT) scans] for more accurate preoperative planning. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:443–452, 2008     

Hip Abduction Can Be Considered the Sole Posterior Precaution Strategy to Lower the Rate of Impingement After Posterior Approach Total Hip Arthroplasty With Large Femoral Head: A Computer Simulation Study

BackgroundStudies suggest that posterior hip precautions are unnecessary after total hip arthroplasty; however, many surgeons and patients choose to follow these precautions to some extent. In this study, we hypothesized that 20° of hip abduction would be sufficient to prevent impingement and dislocation in motions requiring hip flexion when using larger prosthetic heads (≥36 mm) when the acetabular implant is placed within a reasonable orientation (anteversion:15-25° and inclination: 40-60°).MethodsUsing a robotic hip platform, we investigated the effect of hip abduction on prosthetic and bony impingement in 43 patients. For the flexed seated position, anterior pelvic tilt angles of 10 and 20° were chosen, while anterior pelvic tilt angles of 70 and 90° were chosen for the bending forward position. An additional 10° of hip external rotation and 10 or 20° of hip internal rotation were also added to the simulation. One hip received a 32-mm head; otherwise, 36-, 40-mm, or dual-mobility heads were used. The study power was 0.99, and the effect size was 0.644.ResultsIn 65% of the cases, bone-bone impingement between the calcar and anterior-inferior iliac spine was the main type of impingement. The absolute risk of impingement decreased between 0 and 16.3% in both tested positions with the addition of 20° hip abduction.ConclusionWith modern primary total hip arthroplasty stems (low neck diameter) and an overall acceptable cup anteversion angle, small degrees of hip abduction may be the only posterior hip precaution strategy required to lower the risk of dislocation among patients. Future studies can potentially investigate the concept of personalized hip precautions based on preoperative computer simulations, utilized implants, hip-spine relations, and final implant orientation.     

The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios

BACKGROUND: Prosthetic impingement due to poor positioning can limit the range of motion of the hip after total hip arthroplasty. In this study, a computer model was used to determine the effects of the positions of the acetabular and femoral components and of varying head-neck ratios on impingement and range of motion. METHODS: A three-dimensional generic hip prosthesis with a hemispherical cup, a neck diameter of 12.25 millimeters, and a head size ranging from twenty-two to thirty-two millimeters was simulated on a computer. The maximum range of motion of the hip was measured, before the neck impinged on the liner of the cup, for acetabular abduction angles ranging from 35 to 55 degrees and acetabular and femoral anteversion ranging from 0 to 30 degrees. Stability of the hip was estimated as the maximum possible flexion coupled with 10 degrees of adduction and 10 degrees of internal rotation and also as the maximum possible extension coupled with 10 degrees of external rotation. The effects of prosthetic orientation on activities of daily living were analyzed as well. RESULTS: Acetabular abduction angles of less than 45 degrees decreased flexion and abduction of the hip, whereas higher angles decreased adduction and rotation. Femoral and acetabular anteversion increased flexion but decreased extension. Acetabular abduction angles of between 45 and 55 degrees permitted a better overall range of motion and stability when combined with appropriate acetabular and femoral anteversion. Lower head-neck ratios decreased the range of motion that was possible without prosthetic impingement. The addition of a modular sleeve that increased the diameter of the femoral neck by two millimeters decreased the range of motion by 1.5 to 8.5 degrees, depending on the direction of motion that was studied. CONCLUSIONS: There is a complex interplay between the angles of orientation of the femoral and acetabular components. Acetabular abduction angles between 45 and 55 degrees, when combined with appropriate acetabular and femoral anteversion, resulted in a maximum overall range of motion and stability with respect to prosthetic impingement. CLINICAL RELEVANCE: During total hip arthroplasty, acetabular abduction is often constrained by available bone coverage, while femoral anteversion may be dictated by the geometry of the femoral shaft. For each combination of acetabular abduction and femoral anteversion, there is an optimum range of acetabular anteversion that allows the potential for a maximum range of motion without prosthetic impingement after total hip arthroplasty. These data can be used intraoperatively to determine optimum position.     

Length changes of the hamstrings and adductors resulting from derotational osteotomies of the femur.

Derotational osteotomies of the femur are frequently performed to treat persons with cerebral palsy who walk with excessive internal rotation of the hip. However, whether these procedures stretch or slacken the surrounding muscles appreciably is unknown. Determination of how muscle lengths are altered by derotational osteotomies is difficult because the length changes depend not only on the osteotomy site and the degree of derotation, but also on the anteversion angle of the femur and the rotational position of the hip. We have developed a three-dimensional computer simulation of derotational osteotomies, tested by anatomical experiments, to examine how femoral anteversion, hip internal rotation, and derotation affect the lengths of the semitendinosus, semimembranosus, biceps femoris long head, adductor longus, adductor brevis, and gracilis muscles. Simulation of derotational osteotomies at the intertrochanteric, subtrochanteric, or supracondylar levels decreased the origin-to-insertion lengths of the hamstrings and gracilis in our model by less than 8 mm (1.8%). Hence, the lengths of the hamstrings and gracilis are not likely to be altered substantially by these procedures. The origin-to-insertion lengths of the adductor longus and adductor brevis decreased less than 4 mm (1.9%) with subtrochanteric correction in our model, but the length of adductor brevis increased 8 mm (6.3%) with 60 degrees of intertrochanteric derotation. These muscles are also unlikely to be affected by derotational osteotomies, unless a large degree of intertrochanteric derotation is performed.     

In vivo kinematics,component alignment and hardware variables influence on the liner-to-neck clearance during chair-rising after total hip arthroplasty

BackgroundThere is an interest in quantifying dynamic hip kinematics before and after total hip arthroplasty (THA) during chair-rising: one of daily life activities.MethodsThe study consisted of 21 patients who underwent unilateral total hip arthroplasty for symptomatic osteoarthritis. We obtained continuous radiographs using a flat-panel X-ray detector while the participants rose from chair. We assessed the pre and postoperative hip joint's movements using three-dimensional-to-two-dimensional model-to-image registration techniques. We also measured minimum liner-to-neck distances at maximum hip flexion and extension as anterior and posterior liner-to-neck distances, respectively. Multivariate analyses were applied to determine which factors were associated with liner-to-neck distances.ResultsThe cup inclination, cup anteversion, and stem anteversion averaged 37.4°, 23.1°, and 30.1°, respectively. Significantly larger maximum hip flexion angle (72°) was found during chair-rising after THA compared to that before THA (63°, P < 0.01). The anterior pelvic tilt at the maximum hip flexion after THA (3° of anterior tilt) was significantly (P < 0.05) anterior compared to that before THA (1° of posterior tilt). The anterior and posterior liner-to-neck distances averaged 12.3 mm and 8.1 mm, respectively, with a significant difference (P < 0.01). No liner-to-neck contact was found in any hips. In multivariate analysis, the hip flexion angle, cup inclination, stem anteversion and head diameter were significantly associated with the anterior liner-to-neck distance (P < 0.05), the hip extension angle, cup anteversion, neck length and with or without elevated rim were significantly associated with the posterior liner-to-neck distance (P < 0.05, 0.01, 0.05, 0.01, respectively).ConclusionThis study indicates that well-positioned THA provide increased range of hip flexion with sufficient anterior liner-to-neck clearance during chair-rising. Dynamic hip kinematics, component position, and hardware variables significantly influenced on the liner-to-neck clearance under weight-bearing conditions.     

Muscle activation at the human knee during isometric flexion-extension and varus-valgus loads

We examined the role of muscles in counteracting static loads in the transverse plane at the knee to determine if (a) knee muscles are activated to counteract isometric varus or valgus loads, (b) muscle activity during varus and valgus loads changes with the angle of knee flexion, and (c) the direction of a muscle's activation can be predicted by its moment arm orientations. For seven subjects, muscle activity was recorded during isometric tasks using surface and intramuscular electrodes from 10 muscles that span the knee. A six-degree-of-freedom load cell was rigidly attached to each subjects lower leg just above the ankle, and the subjects were instructed to push against the load cell so as to produce moments in the flexion-extension-varus-valgus plane at the knee. Moments in this plane were all of equal magnitude and varied in direction the full 360° in 20° increments. Most muscles were not activated to stabilize the knee against varus-valgus loads, but the sartorius, gracilis, and tensor fasciae latae showed substantial electromyographic activation in these directions. The load directions where muscles were principally active were observed to be dependent on joint angle for some muscles. In particular, the principal directions of activation for these three muscles changed as the angle of knee flexion changed. Similarly, a muscle's moment arm orientation was a good predictor of direction of activation for some muscles and a poor one for others. These results suggest that different muscles may play different roles in providing joint stability and that these roles are complex functions of muscle moment arm orientations, joint angles, external load directions possibly and other undetermined parameters.     

Anteversion angle of the femoral neck in congenital dislocation of the hip

The anteversion angle of femoral neck in 40 children, aged 18 months to 14 years (4.6 years on average), with congenital dislocation of hip was measured pre-operatively on X-ray films according to the way of Dr. Hu's method, and from the proximal femur exposed during operation. The mean value of them was 64.6 degrees and 44 degrees respectively, being statistically significant (P less than 0.01). The method of direct measurement of the anteversion angle during operation was described. The factors affecting the diversity of the measured angle value with different methods were discussed. It seems that the calculation formula used in Hu's method should somehow be corrected and a revision is put forward.