Reverse total shoulder arthroplasty (RTSA) is an increasingly common treatment for osteoarthritic shoulders with irreparable rotator cuff tears. Although very successful in alleviating pain and restoring some function, there is little objective information relating geometric changes imposed by the reverse shoulder and arm function, particularly the moment generating capacity of the shoulder muscles. Recent modeling studies of reverse shoulders have shown significant variation in deltoid muscle moment arms over a typical range of humeral offset locations in shoulders with RTSA. The goal of this study was to investigate the sensitivity of muscle moment arms as a function of varying the joint center and humeral offset in three representative RTSA subjects that spanned the anatomical range from our previous study cohort. We hypothesized there may exist a more beneficial joint implant placement, measured by muscle moment arms, compared to the actual surgical implant configuration. A 12 degree of freedom, subject-specific model was used to represent the shoulders of three patients with RTSA for whom fluoroscopic measurements of scapular and humeral kinematics during abduction had been obtained. The computer model used subject-specific in vivo abduction kinematics and systematically varied humeral offset locations over 1521 different perturbations from the surgical placement to determine moment arms for the anterior, lateral and posterior aspects of the deltoid muscle. The humeral offset was varied from its surgical position ±4 mm in the anterior/posterior direction, ±12mm in the medial/lateral direction, and −10 mm to 14 mm in the superior/inferior direction. The anterior deltoid moment arm varied up to 20 mm with humeral offset and center of rotation variations, primarily in the medial/lateral and superior/inferior directions. Similarly, the lateral deltoid moment arm demonstrated variations up to 20 mm, primarily with humeral offset changes in the medial/lateral and anterior/posterior directions. The posterior deltoid moment arm varied up to 15mm, primarily in early abduction, and was most sensitive to changes of the humeral offset in the superior/inferior direction. The goal of this study was to assess the sensitivity of the deltoid muscle moment arms as a function of joint configuration for existing RTSA subjects. High variations were found for all three deltoid components. Variation over the entire abduction arc was greatest in the anterior and lateral deltoid, while the posterior deltoid moment arm was mostly sensitive to humeral offset changes early in the abduction arc. Moment arm changes of 15–20 mm represent a significant amount of the total deltoid moment arm. This means there is an opportunity to dramatically change the deltoid moment arms through surgical placement of the joint center of rotation and humeral stem. Computational models of the shoulder may help surgeons optimize subject-specific placement of RTSA implants to provide the best possible muscle function, and assist implant designers to configure devices for the best overall performance

Reverse total shoulder arthroplasty (RTSA) is an increasingly common treatment for osteoarthritic shoulders with irreparable rotator cuff tears. Although very successful in alleviating pain and restoring some function there is little objective information relating geometric changes imposed by the reverse shoulder and the moment generating capacity of the shoulder muscles. Recent modeling studies of reverse shoulders have shown significant variation in deltoid muscle moment arms over varied joint centers for shoulders with RTSA. The goal of this study was to investigate the sensitivity of muscle moment arms as a function of varying the joint center in one representative RTSA subject. We hypothesized there may exist a more beneficial joint implant placement, measured by muscle moment arms, compared to the actual surgical implant placement. A 12 degree of freedom, subject-specific model was used to represent the shoulder of a patient with RTSA for whom fluoroscopic measurements of scapular and humeral kinematics during abduction had been obtained. The computer model used these abduction kinematics and systematically varied joint center locations over 1521 different perturbations from the surgical placement to determine moment arms for the anterior, lateral and posterior aspects of the deltoid muscle. The joint center was varied from its surgical position ±4 mm in the anterior/posterior direction, 0–24 mm in the medial/lateral direction, and −10 mm to 14 mm in the superior/inferior direction. The anterior deltoid moment arm varied up to 16mm with center of rotations variations, primarily in the medial/lateral and superior/inferior directions (Figure 2, Table 1(Figure 1)). Similarly, the lateral deltoid moment arm demonstrated variations up to 13 mm, primarily with joint center changes in the anterior/posterior and superior/inferior directions. The posterior deltoid moment arm varied up to 10mm, primarily in early abduction, and was most sensitive to changes of the joint center in demonstrated a sensitivity of 6 mm corresponding to variations in the superior/inferior directions (Figure 2). The goal of this study was to assess the sensitivity of the deltoid muscle moment arms as a function of joint configuration for an existing RTSA subject. High variations were found for all three deltoid components. Variation over the entire abduction arc was greatest in the anterior and lateral deltoid, while the posterior deltoid moment arm was mostly sensitive to joint center changes early in the abduction arc. Moment arm changes of 10–16mm represent a significant amount of the total deltoid moment arm. This means there is an opportunity to dramatically change the deltoid moments arms through surgical placement of the joint center of rotation. Computational models of the shoulder may help surgeons optimize subject-specific placement of RTSA implants to provide the best possible muscle function, and assist implant designers to configure devices for the best overall performance

The Stanford Upper Extremity Model (SUEM) (Holzbauer, Murray, Delp 2005, Ann Biomed Eng) includes the major muscles of the upper limb and has recently been described in scientific literature for various biomechanical purposes including modeling the muscle behavior after shoulder arthroplasty (Hoenecke, Flores-Hernandez, D'Lima 2014, J Shoulder Elbow Surg; Walker, Struk, Banks 2013, ISTA Proceedings). The initial publication of the SUEM compared the muscle moment arm predictions of the SUEM against various moment arm studies and all with the scapula fixed. A more recent study (Ackland, Pak, and Pandy 2008, J Anat) is now available that can be used to compare SUEM moment arm predictions to cadaver data for similar muscle sub-regions, during abduction and flexion motions, and with simulated scapular motion. SUEM muscle moment arm component vectors were calculated using the OpenSim Analyze Tool for an idealized abduction and an idealized flexion motion from 10° to 90° that corresponded to the motions described in Ackland for the cadaver arms. The normalized, averaged muscle moment arm data for the cadavers was manually digitized from the published figures and then resampled into uniform angles matching the SUEM data. Standard deviations of the muscle moment arms from the cadaver study were calculated from source data provided by the study authors. Python code was then used to calculate the differences, percent differences, and root-mean-square (RMS) values between the data sets. Of the 14 muscle groups in the SUEM, the smallest difference in predicted and measured moment arm was for the supraspinatus during the abduction task, with an RMS of the percent difference of 11.4%. In contrast, the middle latissimus dorsi had an RMS percent difference over 400% during the flexion task. The table presents the RMS difference and the RMS of the percent difference for the muscles with the largest abduction and adduction moment arms (during abduction) and the largest flexion and extension moment arms (during flexion). The moment arm data for the SUEM model and the cadaver data (with 1 standard deviation band) during the motion of the same muscles are provided in Figure 1 for the Abduction motion task and in Figure 2 for the Flexion motion task. It is challenging to simulate the three dimensional, time variant geometries of shoulder muscles while maintaining model fidelity and optimizing computational cost. Dividing muscles in to sub regions and using wrapping line segment approximations appears a reasonable strategy though more work could improve model accuracy especially during complex three dimensional motions

Introduction: To ascertain the optimal functional abductor moment arm of the hip for THA, we focused on the Trendelenburg sign. We investigate the various conditions associated with abductor moment arm to achieve a negative Trendelenburg sign postoperatively. Methods: We reviewed 30 patients (34 uncemented primary THAs; mean age, 56 years) at a minimum of two years postoperatively. The tilting angle of the pelvis while performing the Trendelenburg test (Trendelenburg angle) was measured using a magnetic sensor system. On the hip radiographs, the %FO was calculated by dividing the femoral offset, by the distance between the centers of the bilateral femoral heads, and by multiplying by 100. Results: The Trendelenburg angle averaged −4.3 degrees (−9 to −2.0) in all cases with a positive Trendelenburg sign, whereas it averaged +1.4 degrees (−2.0 to +12.0) when the Trendelenburg sign was negative. The %FO having a positive Trendelenburg sign (16.9 %; 10.0 to 22.5) were significantly shorter than those having a negative Trendelenburg sign (19.4 %; 13.5 to 24.7). The Trendelenburg angle correlated positively with %FO. Of the cases with a %FO value of more than 20%, about eighty percent of the cases had a negative Trendelenburg sign. Discussion: The optimum abductor moment-arm, when the Trendelenburg sign is negative, has not been described. This study indicates that a %FO of 20 % may be one of the factors taken into account when determining the suitable size and position for acetabular and femoral neck components

Abstract. Objectives. The need for gender specific knee arthroplasty is debated. This research aimed to establish whether gender differences in patellar tendon moment arm (PTMA), a composite measure that characterises function of both the patellofemoral and tibiofemoral joints, are a consequence of knee size or other variation. Methods. PTMA about the instantaneous helical axis was calculated from positional data acquired using optical tracking. First, data post-processing was optimised, comparing four smoothing techniques (raw, Butterworth filtered, generalised cross-validation cubic spline interpolated and combined filtered/interpolated) using a fabricated knee. Then PTMA was measured during open-chain extension for N=24 (11 female) fresh-frozen cadaveric knees, with physiologically based loading and extension rates (420°/s) applied. Gender differences in PTMA were assessed before and after accounting for knee size with epicondylar width. Results. Combined smoothing enabled sub-mm accuracy (root-mean-squared (RMS) error 0.16mm, max error 0.47mm), whereas large errors were measured for raw (RMS 3.61mm, max 23.71mm), filtered-only (RMS 1.19mm, max 7.38mm) and interpolated-only (RMS 0.68mm, max 1.80mm) techniques. Before scaling, average PTMA throughout knee flexion was 46mm and mean, maximum, and minimum absolute values of PTMA were larger in males (mean differences >8mm, p<0.001), as were the PTMAs at terminal extension and flexion, and the change in PTMA from peak to terminal extension (differences >4mm, p<0.05). After scaling, the PTMA in deep flexion and the change in PTMA from peak to terminal extension were still larger in male knees (differences >2mm, p<0.05). The flexion angle of peak PTMA, unaffected by scaling, was closer to terminal extension for female knee (female 15°, male 29°, p<0.05). Conclusion. Gender differences in PTMA were identified both before and after accounting for knee size, with implications for gender-specific arthroplasty and musculoskeletal models. The developed measurement framework could also be applied in vivo for accurate measurement of the PTMA. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Introduction: The goals of TKR are restoration of the mechanical axis, joint line and Q –angle. Reproduction of the offset of the extensor mechanism during arthroplasty is less well understood. The lever arm of the extensor mechanism is primarily affected by femoral component position, patella tracking and overall patellar thickness. Changing this lever arm alters quadriceps muscle and patellofemoral joint reaction force. Some TKR designs purposefully aim to increase this offset in order to reduce PFJ contact pressures. Overstuffing the PFJ will however adversely affect outcome. The aims of this study were to measure and compare the pre and postoperative quadriceps lever arm and its effect on function in a consecutive series of patients undergoing TKR. Methods: Fifty consecutive patients who had an LCS TKR without patellar resurfacing by a single surgeon were reviewed. Patient demographics were recorded. We measured the pre and post-operative quadriceps lever arm using comparable lateral radiographs and digital imaging software. Functional outcome in these patients was determined using the American Knee Society Score pre and 6-months post-operatively. Patients were specifically questioned regarding the presence or absence of anterior knee pain. Results: Quadriceps lever arm was on average 6 mm greater post-operatively than pre-op but this difference was not significant [t-test]. The functional outcome in these patients was independent of any change in quadriceps lever arm. Conclusions Reproduction of normal biomechanics is essential in order to optimise outcome from TKR. Using the LCS system, there is a small but insignificant increase in the quadriceps moment arm. We have not found that this has any bearing on functional outcome in these patients

As treatments of knee osteoarthrosis are continually refined, increasingly sophisticated methods of evaluating their biomechanical function are required. Whilst TKA shows good preoperative pain relief and survivorship, functional outcomes are sub-optimal, and research focus has shifted towards their improvement. Restoration of physiological function is a common design goal that relies on clear, detailed descriptions of native biomechanics. Historical simplifications of true biomechanisms, for example sagittal plane approximation of knee kinematics, are becoming progressively less suitable for evaluation of new technologies. The patellar tendon moment arm (PTMA) is an example of such a metric of knee function that usefully informs design of knee arthroplasty but is not fully understood, in part due to limitations in its measurement. This research optimized PTMA measurement and identified the influence of knee size and sex on its variation. The PTMA about the instantaneous helical axis was calculated from optical tracked positional data. A fabricated knee model facilitated calculation optimization, comparing four data smoothing techniques (raw, Butterworth filtering, generalized cross-validated cubic spline-interpolation and combined filtering/interpolation). The PTMA was then measured for 24 fresh-frozen cadaveric knees, under physiologically based loading and extension rates. Sex differences in PTMA were assessed before and after size scaling. Large errors were measured for raw and interpolated-only techniques in the mid-range of extension, whilst both raw and filtered-only methods saw large inaccuracies at terminal extension and flexion. Combined filtering/interpolation enabled sub-mm PTMA calculation accuracy throughout the range of knee flexion, including at terminal extension/flexion (root-mean-squared error 0.2mm, max error 0.5mm) (Figure 1). Before scaling, mean PTMA throughout flexion was 46mm; mean, peak, and minimum PTMA values were larger in males, as was the PTMA at terminal flexion, the change in PTMA from terminal flexion to peak, and the change from peak to terminal extension (mean differences ranging from 5 to 10mm, p<0.05). Knee size was highly correlated with PTMA magnitude (r>0.8, p<0.001) (Figure 2). Scaling eliminated sex differences in PTMA magnitude, but peak PTMA occurred closer to terminal extension in females (female 15°, male 29°, p=0.01) (Figure 3). Improved measurement of the PTMA reveals previously undocumented characteristics that may help to improve the functional outcomes of knee arthroplasty. Knee size accounted for two-thirds of the variation in PTMA magnitude, but not the flexion angle at which peak PTMA occurred, which has implications for morphotype-specific arthroplasty and musculoskeletal models. The developed calculation framework is applicable both in vivo and vitro for accurate PTMA measurement and might be used to evaluate the relative performance of emerging technologies. For any figures or tables, please contact the authors directly

Differences at motor control strategies to provide dynamic balance in various tasks in diabetic polyneuropatic (DPN) patients due to losing the lower extremity somatosensory information were reported in the literature. It has been stated that dynamics of center of mass (CoM) is controlled by center of pressure (CoP) during human upright standing and active daily movements. Indeed analyzing kinematic trajectories of joints unveil motor control strategies stabilizing CoM. Nevertheless, we hypothesized that imbalance disorders/CoM destabilization observed at DPN patients due to lack of tactile information about the base of support cannot be explained only by looking at joint kinematics, rather functional foot usage is proposed to be an important counterpart at controlling CoM. In this study, we included 14 DPN patients, who are diagnosed through clinical examination and electroneuromyography, and age matched 14 healthy subjects (HS) to identify control strategies in functional reach test (FRT). After measuring participants’ foot arch index (FAI) by a custom-made archmeter, they were tested by using a force plate, motion analysis system, surface electromyography and pressure pad, all working in synchronous during FRT. We analyzed data to determine effect of structural and functional foot pathologies due to neuropathy on patient performance and postural control estimating FAI, reach length (FR), FR to height (H) ratio (FR/H; normalized FR with respect to height), displacement of CoM and CoP in anteroposterior direction only, moment arm (MA, defined as the difference between CoP and CoM at the end of FRT), ankle, knee and hip joint angles computed at the sagittal plane for both extremities. Kinematic metrics included initial and final joint angles, defined with respect to start and end of reaching respectively. Further difference in the final and initial joint angles was defined as Δ. FAI was founded significantly lower in DPN patients (DPN: 0.3404; HS: 0.3643, p= <0.05). The patients’ FR, FR/H and absolute MA and displacement of CoM were significantly shorter than the control group (p= <0.05). Displacement of CoP between the two groups were not significant. Further we observed that CoM was lacking CoP in DPN patients (mean MA: +0.88 cm), while leading CoP in HS (mean MA: −1.59 cm) at the end of FRT. All initial angles were similar in two groups, however in DPN patients final right and left hip flexion angle (p=0.016 and p=0.028 respectively) and left ankle plantar flexion angle (p=0.04) were smaller than HS significantly. DPN patients had significantly less (p=0.029) hip flexion (mean at right hip angle, Δ=25.0°) compared to HS (Δ=33.53°) and ankle plantar flexion (DPN mean at right ankle angle, Δ=6.42°, HS mean Δ=9.07°; p=0.05). The results suggest that movement of both hip and ankle joints was limited simultaneously in DPN patients causing lack of CoM with respect to CoP at the end of reaching with significantly lower FAI. These results lead to the fact that cutaneous and joint somatosensory information from foot and ankle along with the structure of foot arch may play an important role in maintaining dynamic balance and performance of environmental context. In further studies, we expect to show that difference at control strategies in DPN patients due to restricted functional foot usage might be a good predictor of how neuropathy evolves to change biomechanical aspects of biped erect posture

Anterior cruciate ligament reconstruction (ACLR) using a semitendinosus (ST) autograft, with or without gracilis (GR), results in donor muscle atrophy and varied tendon regeneration. The effects of harvesting these muscles on muscle moment arm and torque generating capacity have not been well described. This study aimed to determine between-limb differences (ACLR vs uninjured contralateral) in muscle moment arm and torque generating capacity across a full range of hip and knee motions. A secondary analysis of magnetic resonance imaging was undertaken from 8 individuals with unilateral history of ST-GR ACLR with complete ST tendon regeneration. All hamstring muscles and ST tendons were manually segmented. Muscle length (cm), peak cross-sectional area (CSA) (cm. 2. ), and volume (cm. 3. ) were measured in ACLR and uninjured contralateral limbs. OpenSim was used to simulate and evaluate the mechanical consequences of changes in normalised moment arm (m) and torque generating capacity (N.m) between ACLR and uninjured contralateral limbs. Compared to uninjured contralateral limbs, regenerated ST tendon re-insertion varied proximal (+) (mean = 0.66cm, maximum = 3.44cm, minimum = −2.17cm, range = 5.61cm) and posterior (+) (mean = 0.38cm maximum = 0.71cm, minimum = 0.02cm, range = 0.69cm) locations relative to native anatomical positions. Compared to uninjured contralateral limbs, change in ST tendon insertion point in ACLR limbs resulted in 2.5% loss in peak moment arm and a 3.4% loss in peak torque generating capacity. Accounting for changes to both max isometric force and ST moment arm, the ST had a 14.8% loss in peak torque generating capacity. There are significant deficits in ST muscle morphology and insertion points following ST-GR ACLR. The ST atrophy and insertion point migration following ACLR may affect force transmission and distribution within the hamstrings and contribute to persistent deficits in knee flexor and internal rotator strength

Reverse total shoulder arthroplasty (RTSA) is increasingly used in the United States since approval by the FDA in 2003. RTSA relieves pain and restores mobility in arthritic rotator cuff deficient shoulders. Though many advantages of RTSA have been demonstrated, there still are a variety of complications (implant loosening, shoulder impingement, infection, frozen shoulder) making apparent much still is to be learned how RTSA modifies normal shoulder function. The goal of this study was to assess how RTSA affects deltoid muscle moment generating capacity post-surgery using a subject-specific computational model driven by in vivo kinematic data. A subject-specific 12 degree-of-freedom (DOF) musculoskeletal model was used to analyze the shoulders of 27 subjects (14-RTSA, 12-Normal). The model was modified from the work of Holzbaur et al. to directly input 6 DOF humerus and scapula kinematics obtained using fluoroscopy. Model geometry was scaled according to each subject's skeletal dimensions. In vivo abduction kinematics for each subject were input to their subject-specific model and muscle moment arms for the anterior, lateral and posterior aspects of the deltoid were measured over the arc of motion. Similar patterns of muscle moment arm changes were observed for normal and RTSA shoulders. The moment arm of the anterior deltoid was positive with the arm at the side and decreased monotonically, crossing zero (the point at which the muscle fibers pass across the joint center) between 50°–60° glenohumeral abduction (Figure 1a). The average moment arm of the lateral deltoid was constant and positive in normal shoulders, but showed a decreasing trend with abduction in RTSA shoulders (Figure 1b). The posterior deltoid moment arm was negative with the arm at the side, and increased monotonically to a positive value with increasing glenohumeral abduction (Figure 1c). Subject-specific moment arm values for RTSA shoulders were highly variable compared to normal shoulders. 2-way repeated measures ANOVA showed significant differences between RTSA and normal shoulders for all three aspects of the deltoid moment arm, where the moment arms in RTSA shoulders were smaller in magnitude. Shoulder functional capacity is a product of the moment generating ability of the shoulder muscles which, in turn, are a function of the muscle moment arms and muscle forces. Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and, therefore, the muscle moment arms in the RTSA shoulder. Our results show RTSA shoulders maintain the same muscle moment arm patterns as healthy shoulders, but they show much greater inter-subject variation and smaller moment arm magnitudes. These observations show directly how RTSA configuration and implant placement affect deltoid moment arms, and provide an objective basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner

Introduction. Quadriceps weakness, which is often reported following total knee arthroplasty (TKA), affects patients' abilities to perform activities of daily living [1]. Implant design features, particularly of the patella-femoral joint, influence the mechanical advantage of the extensor mechanism. This study quantifies the changes in extensor mechanism moment arms due to different patellar resurfacing options during TKA. Methods. Posterior-stabilized TKR surgery was performed on seven cadaveric knees which were subsequently mounted in the Kansas Knee Simulator (KKS) [2]. A dynamic physiological squat was simulated between 5° and 80° knee flexion at 50% body weight while knee kinematics, including the lines of action of the rectus femoris (RF) muscle and patellar tendon (PT), were recorded using an optical tracking system. The simulation was performed after three patella treatment options: 1) leaving the native patella Unresurfaced, 2) resurfaced with a medialized Dome patella, and 3) resurfaced with a medialized Anatomic patella which included a conforming lateral facet. Moment arms from the tibio-femoral helical axis to the line of action of the PT and the RF were calculated for each patella condition. Results. The quadriceps moment arm for the Anatomic patella is smaller than the Dome during extension (Fig. 1A). Past 55° the Anatomic moment arm becomes larger than the dome. Patellar tendon moment arm for Anatomic is bigger than the dome in extension (0–50°) but smaller in flexion (50–80°) (Fig. 1B). The overall shape of the Unresurfaced patella moment arm through flexion, for both the patellar tendon and the quadriceps, was more similar to the Anatomic than the Dome although the difference in magnitude was not consistent between the six knees. Discussion. The orientations of both the RF and PT lines of action, which were used to determine the moment arm, were correlated with patellofemoral kinematics. A more extended position of the patella resulted in an increase of the PT while decreased the RF moment arm. This explains the difference between the Anatomic and dome moment arms for both PT and RF since the Anatomic patella was more extended between 0–60° knee flexion (Fig. 1C). The similarity in the PT and RF moment arms shape between the Anatomic and the Unresurfaced resurfaced was due to the similarity in their conforming geometry. The less conforming geometry of the Dome patella made it less constrained and allowed the forces applied by the RF and PT to have a greater influence on patellofemoral kinematics and moment arms than Anatomic and Unresurfaced patellae. The small changes in PT and RF moment arm observed in this study can result in large effect on muscle loads that are required to perform more strenuous activities. Multiple methodologies have been reported in literature to calculate moment arm. Future work will examine the effects of different methodologies on moment arm calculations as well as validation of results by examining the change in quadriceps moment arm required to perform certain activities

Background:. An upper extremity model of the shoulder was developed from the Stanford upper extremity model (Holzbaur 2005) in this study to assess the muscle lengthening changes that occur as a function of kinematics for reverse total shoulder athroplasty (RTSA). This study assesses muscle moment arm changes as a function of scapulohumeral rhythm (SHR) during abduction for RTSA subjects. The purpose of the study was to calculate the effect of RTSA SHR on the deltoid moment arm over the abduction activity. Methods:. The model was parameterized as a six degree of freedom model in which the scapula and humeral rotational degrees of freedom were prescribed from fluoroscopy. The model had 15 muscle actuators representing the muscles that span the shoulder girdle. The model was then uniformly scaled according to reflective markers from motion capture studies. An average SHR was calculated for the normal and RTSA cohort set. The SHR averages were then used to drive the motion of the scapula and the humerus. Lastly 3-dimensional kinematics for the scapula and humerus from 3d-2d fluoroscopic image registration techniques were used to drive the motion of model. Deltoid muscle moment arm was calculated. Results:. Muscle moment arms were calculated for the anterior, lateral and posterior heads of the deltoid. Significant changes (>1 mm) were only found in comparing the anterior deltoid muscle moment arm predictions between the normal and RTSA group. The anterior deltoid for RTSA had a moment arm range from −12.5–20.6 mm over the max abduction arc. The anterior deltoid for normal group had a moment arm range from −14.5–22.6 mm over the max abduction arc. There is a difference of 2 mm between the normal and RTSA anterior deltoid moment arm that converges to 0 at 45° of elevation. The 2 mm difference is also seen again as the difference diverges again (Figure 1). There were no significant differences found between normal and RTSA groups for the lateral and posterior deltoid. The most significant difference between moment arm calculations for the RTSA and normal group was found in the Anterior deltoid. (Figure 1). Conclusion:. It was found that the muscle moment arms in the RTSA group were significantly different than in the normal group for the anterior deltoid. No other significant differences were found. In the initial 40° of elevation there is a 2 mm difference in anterior deltoid muscle moment arm between the normal and RTSA group. This difference is also found is seen from 60°–90° of elevation. From 35° −55° there is no difference between RTSA and normal groups. SHR for the RTSA (1.8: 1) is significantly lower than in the normal (2.5: 1) group. Differences found in muscle moment arms over the abduction arc between RTSA and normal groups point to the significant change of the anterior deltoid after RTSA. This study primary objective was to assess the differences in muscle moment arms as a function of SHR (Kinematic differences). Significant differences found may improve implant design, surgical technique, and rehabilitative strategies for reverse shoulder surgery

Purpose. To evaluate the clinical and radiologic midterm results of rotational acetabular osteotomy (RAO) in incongruent hip joints. Material and Methods. A consecutive series of 15 hips in 14 patients who underwent RAO in incongruent hip joint were evaluated at an average follow-up of 52.3 months (range from 36 to 101 months). The average age at operation was 27 years (range from 12 to 38 years) old. The preoperative diagnoses were developmental dysplasia in 4 hips, sequelae of Legg-Calvé-Perthes disease in 8 hips, and multiple epiphyseal dysplasia in 3 hips. The RAO procedures were combined with a femoral valgus oseotomy in 10 hips, advance osteotomy of greater trochanter in 4 hips, derotational osteotomy in 2 hips. Clinically, Harris hip score, range of motion, leg length discrepancy(LLD) and hip joint pain were evaluated. Radiological changes of anterior and lateral center-edge(CE) angle, acetabular roof angle, acetabular head index(AHI), ratio of body weight moment arm to abductor moment arm, and a progression of osteoarthritis were analyzed. Results. The Harris hip score ha been improved from average from 67.5 points preoperatively to 97.6 points postoperatively. There have been no significant changes in the range of motion. The anterior CE angle increased from an average of 9.0°(-19.7□18.6°) to 32.5°(22.6□39.1°), the lateral CE angle from 7.6°(-12.1□14.1)° to 31.7°(26.5□37.8°) and the AHI from 61%(33□73%) to 86%(65□100%). The average ratio of body weight moment arm to abductor moment arm was changed 1.88 to 1.49. There was no case showing progression of osteoarthritis. None of the patients experienced revision surgery. Conclusion. The conventional salvage operation, such as Chiari osteotomy, has been recommended in incongruent hip. However, if we can expect to have a congruency after RAO with/without any femoral osteotomies, it would be a hopeful procedure for the incongruent joints by enhancing acetabular coverage, taking joint surface with normal articular cartilage, increasing abductor moment arm with additional improvement in LLD

Summary Statement. Reverse shoulder design philosophy can impact external rotation moment arms. Lateralizing the humerus can increase the external rotator moment arms relative to normal anatomy. Introduction. The design of reverse shoulders continues to evolve. These devices are unique in that they are not meant to reproduce the healthy anatomy. The reversal of the fulcurm in these devices impacts every muscle that surrounds the joint. This study is focused on analyzing the moment arms for the rotator cuff muscles involved in internal and external rotation for a number of reverse shoulder design philosophies. Methods. Four of the most common design philosophies were chosen. The first, a Grammont style prosthesis, with a center of rotation (COR) on the glenoid face and a humeral cup countersunk into the proximal humerus (MGMH). The second concept is the MGMH design lateralised by a 10mm bone graft (BIO). The third concept has a lateralised glenosphere COR and a humeral component inside the proximal humerus (LGMH). The fourth design has a medialised COR with a humeral component placed on top of the humerus (MGLH). This places the humerus further lateral than the previous designs. For each component set, a representative implant was modeled based on published specifications. Each design was implanted into the same digital bone models (consisting of a humerus, scapula, clavicle, and ribcage) following the manufacturer's recommended surgical technique. The muscles analyzed were the posterior-deltoid (PD), subscapularis (SSC), infraspinatus (IS), and teres minor (TM). These muscles were allowed to wrap around the bone of the scapula and proximal humerus through the range of motion. All muscle origin and insertion points were kept constant throughout the analysis. The assemblies were externally rotated from an initial position of 45° internal rotation to 45° of external rotation of the humerus with the arm at 0° of abduction. The moment arms for all muscles were compared to those calculated for the anatomic shoulder. Results. All the rotator cuff muscles displayed a similar trend with the reverse shoulder. The external rotators all had similar moment arm values at neutral (IS∼22mm, TM∼20mm), but increased at rates proportional to their humeral offsets with external rotation (IS-MGLH 32.3mm, LGMH 27.5mm, MGMH and BIO 26.25mm; TM-MGLH 31.3mm, LGMH 27.8mm, MGMH and BIO 26.5mm). The SSC internal rotation moment arm remains roughly constant at 20mm for the anatomic shoulder, but varies widely from 45° external to 45° internal rotation with the different designs (MGLH 31.4mm to 6.7mm; MGMH 25.1mm to 11.2mm; LGMH 26.2mm to 10.8mm; BIO 25.4mm to 4.8mm). The PD moment arm is increased relative to the anatomic shoulder during external rotation for the MGLH design (9.3mm vs. 7.4mm). The other designs exhibit a decrease in the moment arm of this muscle relative to the anatomic design (LGMH 7.3mm, MGMH 5.8mm, BIO 6.4mm). Discussion. The lateral offset between the center of humeral axis and the muscle insertion on the humerus dominates the external rotation moment arm value through this range of motion. This is evident by the increase in the moment arms with external rotation for the different reverse shoulder designs. The increase in external rotation efficiency for the external rotators and PD could play a critical role in post-operative external rotation strength and motion

Background. Though many advantages of reverse total shoulder arthroplasty (RTSA) have been demonstrated, a variety of complications indicate there is much to learn about how RTSA modifies normal shoulder function. This study assesses how RTSA affects deltoid muscle moment arms post-surgery using a subject-specific computational model driven by in vivo kinematic data. Methods. A subject-specific 12 degree-of-freedom (DOF) musculoskeletal model was used to analyze the shoulders of 26 subjects (14 RTSA, 12 Normal). The model was modified from the work of Holzbaur et al. to directly input 6 DOF humerus and scapula kinematics obtained using fluoroscopy. Results. The moment arm of the anterior, lateral and poster aspects of the deltoid was found to be significantly different when comparing RTSA and normal cohorts. Anterior and lateral deltoid moment arms were found to be larger at initial elevation. There was large inter-subject variability within the RTSA group. Conclusion. Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and the muscle moment arms in the RTSA shoulder. RTSA shoulders maintain the same anterior and posterior deltoid muscle moment arm patterns as healthy shoulders, but they show much greater inter-subject variation and larger moment arm magnitudes. These observations provide a basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner

Hip abductor tears(AT) have long been under-recognized, under-reported and under-treated. There is a paucity of data on the prevalence, morphology and associated factors. Patients with “rotator cuff tears of the hip” that are recognized and repaired during total hip arthroplasty(THA) report comparable outcomes to patients with intact abductor tendons at THA. The study was a retrospective review of 997 primary THA done by a single surgeon from 2012–2022. Incidental findings of AT identified during the anterolateral approach to the hip were documented with patient name, gender, age and diagnosis. The extent and size of the tears of the Gluteus medius and Minimus were recorded. Xrays and MRI's were collected for the 140 patients who had AT and matched 1:1 with respect to age and gender against 140 patients that had documented good muscle quality and integrity. Radiographic measurements (Neck shaft angle, inter-teardrop distance, Pelvis width, trochanteric width and irregularities, bodyweight moment arm and abductor moment arm) were compared between the 2 groups in an effort to determine if any radiographic feature would predict AT. The prevalence of AT were 14%. Females had statistically more tears than males(18vs10%), while patients over the age of 70y had statistically more tears overall(19,7vs10,4%), but also more Gluteus Medius tears specifically(13,9vs5,3%). Radiographic measurements did not statistically differ between the tear and control group, except for the presence of trochanteric irregularities. MRI's showed that 50% of AT were missed and subsequently identified during surgery. Abductor tears are still underrecognized and undertreated during THA which can results in inferior outcomes. The surgeon should have an high index of suspicion in elderly females with trochanteric irregularities and although an MRI for every patient won't be feasible, one should always be prepared and equipped to repair the abductor tendons during THA

Introduction: During total hip arthroplasty various femoral stem offsets are available and the femoral stem can be placed in either varus or valgus. The overall effect of this is to increase or decrease the functional offset at the hip joint. Many authors have investigated the effects of the functional offset upon the reconstructed hip joint. To our knowledge no studies have concentrated on the effects, if any, upon the loading and function of the knee joint. The aim of this study was to investigate the effects, if any, of reducing functional offset at the knee. Materials and methods: To study the effects of alterations in functional offset during hip arthroplasty, a biomechanical computer model was constructed. Normal lower limb anthropometric measurements available in the literature were used within this program. The model thus constructed calculated the effect of different functional offsets upon moments about the knee in stationary standing on one leg. The model also allowed for different varus/valgus placement of the stem. Results: Reducing prosthetic neck length reduces the moment arm created by the ground reaction force about the knee axis. Placing the stem in valgus reduces the moment arm. This reduction depends upon the length of the neck. Discussion: In the one legged stance, the line of weight is offset in the coronal plane from the AP axis of the knee joint producing an adducting moment about the knee, balanced by tension in the lateral collateral ligament of the knee and the iliotibial tract. Any reduction in this moment arm would alter the loading of the knee, altering the value of the force on the lateral compartment of the knee. Indeed, it is possible to develop a situation where the knee moment arm creates an abducting moment about the knee, increasing the loading of the lateral compartment of the knee. This may lead to valgus malalignment of the knee

Abstract. Patellofemoral Arthroplasty (PFA) is an alternative to TKA for patellofemoral osteoarthritis that preserves tibiofemoral compartments. It is unknown how implant positioning affects biomechanics, especially regarding the patella. This study analysed biomechanical effects of femoral and patellar component position, hypothesising femoral positioning is more important. Nine cadaveric knees were studied using a repeated-measures protocol. Knees were tested intact, then after PFA implanted in various positions: neutral (as-planned), patellar over/understuffing (±2mm), patellar tilt, patellar flexion, femoral rotation, and femoral tilt (all ±6°). Arthroplasties were implemented with CT-designed patient-specific instrumentation. Anterior femoral cuts referenced Whiteside's line and all femoral positions ensured smooth condyle-to-component transition. Knee extension moments, medial patellofemoral ligament (MPFL) length-change, and tibiofemoral and patellofemoral kinematics were measured under physiological muscle loading. Data were analysed with one-dimensional statistical parametric mapping (Bonferroni-Holm corrected). PFA changed knee function, altering extension moments (p<0.001) and patellofemoral kinematics (p<0.05), but not tibiofemoral kinematics. Patellar component positioning affected patellofemoral kinematics: over/understuffing influenced patellar anterior translation and the patellar tendon moment arm (p<0.001). Medially tilted patellar cuts produced lateral patellar tilt (p<0.001) and vice versa. A similar inverse effect occurred with extended/flexed patellar cuts, causing patellar flexion and extension (p<0.001), respectively. Of all variants, only extending the patellar cut produced near-native extension moments throughout. Conversely, the only femoral effect was MPFL length change between medially/laterally rotated components. PFA can restore native knee biomechanics. Provided anterior femoral cuts are controlled and smooth condyle-to-component transition assured, patellar position affects biomechanics more than femoral, contradicting the hypothesis

Introduction. The pathogenesis of primary knee osteoarthritis is due to excess mechanical loading of the articular cartilage. Previous studies have assessed the impact of muscle forces on tibiofemoral kinematics and force distribution. A cadaveric study was performed to evaluate the effect of altering the moment arm of the iliotibial band (ITB) on knee biomechanics. Method. A robotic system consisting of a 6-DOF manipulator capable of measuring forces on the medial and lateral condyle of a cadaveric knee at various flexion angles and muscle forces was utilized [1]. The system measured the compartment forces at flexion angles between 0° and 30° under 3 simulated loading conditions (300N quadriceps, 100N hamstrings and: i. 0N ITB; ii. 50N ITB; iii. 100N ITB). Eight fresh frozen human cadaver knee specimens (4 males, 4 females); age range 36 – 50 years; weight range 49 – 90 kg; height range 154 – 190 cm were used in the study. The ITB and associated lateral soft tissue structures were laterally displaced from the lateral femoral condyle by fixing a metal implant (like in Figure 1) to the distal lateral femur. Mechanical loads on the medial and lateral compartments (with and without the implant) were measured using piezoelectric pressure sensors. Results. For each specimen, lateral displacement of the ITB due to the implant was measured (15 – 20 mm). The % average unloading of the medial compartment for all the specimens ranged from 34% – 65% (Figure 2). Also observed was a concomitant increase in lateral compartment load. Medial unloading was even observed with no ITB force (0N) which indicates a role for other lateral structures attached to the ITB in unloading the medial compartment [2]. In addition, under these non-weight bearing conditions, on average, there was an increase in valgus tibial angulation through the flexion range. Discussion. Increasing mechanical leverage of muscles across a joint is accomplished in nature through sesamoid bones (e.g., patella) which increase the muscle moment arm. By increasing the moment arm of the ITB and lateral soft tissue structures by lateralizing these structures, our model demonstrates a 34–65% unloading of the medial compartment. Studies of knee braces and weight loss have shown that reducing mechanical load on the medial condyle by even 10% provides clinical benefits in terms of reduced pain and improved function. Based on the results of this study, unloading the medial compartment by displacing the ITB laterally may be a means of treating medial osteoarthritis (Figure 3). A prospective, multi-center, non-randomized, open label, single-arm study is currently underway to establish the safety and efficacy of providing medial osteoarthritis pain relief by displacing the ITB using Cotera, Inc.'s Latella™ Knee Implant

INTRODUCTION:. The 3D shape of the normal proximal femur is poorly described in current designs of proximal femur prosthesis. Research has shown that in current implant designs with small diameter femoral heads the moment arm of the ilio-psoas tendon is reduced causing weakness in full extension, while large femoral heads cause psoas tendon impingement on the femoral head neck junction [1]. The femoral head-neck junction thus directly influences the hip flexor muscles' moment arm. Mathematical modeling of proximal femoral geometry allowed a novel proximal femur prosthesis to be developed that takes into account native anatomical parameters. We hypothesized that it is possible to fit a quadratic surface (e.g. sphere, cylinder…) or combinations of them on different bone surfaces with a relatively good fit. METHODS:. Forty six ‘normal’ hips with no known hip pathology were segmented from CT data. Previous research has shown the femoral head to have a spherical shape [2], the focus here was therefore mainly on the neck. The custom-written minimization algorithm, using least squares approximation methods, was used to optimize the position and characteristics of the quadratic surface so that the sum of distances between a set of points on the femoral neck and the quadratic surface was minimized. Furthermore, to improve upon current design regarding the transition between head and the neck, we recorded the position of the head neck articular margin in addition the slope of the transition from head to neck in the above 46 hips. RESULTS:. The femoral neck was found to be represented with a good fit as a quadratic surface (hyperboloid) with an average root mean square error of 1.0 ± 0.13 among 46 hips. The femoral head was spherical with a mean ratio of 22.6 ± 1.75 mm. The shape of the femoral articular margin is a reproducible sinusoidal wave form, which appears to have two facets, one anterior and the other posterior. A sigmoid curve, provided by the Logistic Function was used to switch smoothly from the spherical head function to the hyperboloid neck function (Fig. 1). This curve provides a continuous mathematical function to describe the head/neck geometry. DISCUSSION:. Traditional designs that liken the femoral head to a sphere are an oversimplification of normal hip morphology. The precise shape of the neck and the relationship of the neck to the head are the basis of this invention. The prosthesis is designed to avoid soft tissue impingement and can be optimised in shape and size to match the patient's native morphology. Neck diameter and length can be designed to achieve the optimum head-neck ratio to further improve the range of motion produced. With the current design the pain observed due to ilio-psoas impingement to implant will be reduced. Furthermore as the implant is anatomical the function of muscles and their moment arm will be unaffected