Choongsoo S. Shin

Aims: The biomechanical effects of varying rotator cuff tear severities on medialized versus lateralized reverse total shoulder arthroplasty (RTSA) remain unclear. This study aimed to compare medialized and lateralized RTSA designs based on subscapularis integrity and the severity of posterosuperior cuff tears. Methods: A total of 12 human in vivo experimental datasets were collected during external rotation (ER) from a neutral position to 45°, with the elbow fixed at 90° and the palm facing inward. These datasets were used as inputs for musculoskeletal shoulder models of both medialized and lateralized RTSA. Inverse dynamic simulations were conducted under two subscapularis conditions—repaired (all bundles intact) and torn (all bundles torn)—across three stages of posterosuperior cuff tear severity. The scapular notching-related impingement stress, joint subluxation, and muscle-tendon forces were compared between the two RTSA configurations. Results: Subscapularis repair in lateralized RTSA led to greater reductions in impingement stress (16.7% to 26.2%; p < 0.05) and joint subluxation (27.5% to 58.9%; p < 0.001) compared to medialized RTSA (14.1% to 18.6%; p < 0.05 and 14.7% to 22.4%; p < 0.001, respectively) across all posterosuperior cuff tear conditions. Additionally, with subscapularis repair, posterior deltoid force increased more markedly with tear severity in lateralized RTSA (from 2.6% (p = 0.007) to 918.5% (p < 0.001)) than in medialized RTSA (from 4.7% to 784.5%, both p < 0.001). Conclusion: Lateralizing RTSA and repairing the subscapularis can significantly reduce scapular notching-related impingement, improve joint stability, and support ER torque generation by increasing posterior deltoid force in patients with posterosuperior cuff tears. These findings provide critical insights for surgical planning, supporting the use of a lateralized implant and subscapularis repair to reduce the risk of scapular notching and joint subluxation while enhancing ER torque generation. Cite this article: Bone Joint Res 2026;15(2):135–147.

The effects of different posterosuperior rotator cuff tear patterns, ranging from isolated supraspinatus tears to partial or complete infraspinatus involvement, on glenohumeral joint contact mechanics in reverse total shoulder arthroplasty (RTSA) remain unclear. This study aimed to investigate how varying levels of posterosuperior cuff tear (PCT) severity affect three-dimensional center-of-pressure (COP) trajectories, joint contact forces (JCFs), and muscle-tendon forces using a musculoskeletal shoulder model of RTSA. Three-dimensional marker trajectory data from eleven male participants were collected during 120° of shoulder abduction in the coronal plane and used as consistent inputs for the RTSA model. Model simulations were performed for various stages of PCT severity. For comparison with each stage of the model, an intact cuff model of RTSA was also simulated. As PCTs progressed, the COP trajectories increasingly shifted in the superior (P < 0.001), anterior (P < 0.001), and medial (P < 0.001) directions. Additionally, the peak inferior and posterior JCFs decreased by 5.6% (P = 0.02) and 20.0% (P = 0.002), respectively (i.e., the contact area moved in a more anterosuperior direction). Conversely, preserving more posterosuperior cuffs increased muscle-tendon forces in the subscapularis and pectoralis major. Preserving the middle-inferior infraspinatus can restore joint contact mechanics and mitigate increased muscle-tendon forces in RTSA. These findings can help orthopedic surgeons in deciding whether to repair the rotator cuff during RTSA to mitigate increased muscle-tendon forces and restore the transverse force couple between the anterior and posterior cuffs, thereby improving glenohumeral joint stability.

Background: Knee valgus loading is thought to be an important contributor to noncontact anterior cruciate ligament (ACL) injuries, but the effects of training programs focusing on decreasing knee valgus loading on lower extremity biomechanics with respect to ACL injury risk remain unclear. Thus, this study aimed to examine the effect of strength training designed to strengthen the medial thigh muscles on lower extremity joint kinematics, kinetics and muscle activity during single-leg landing. Methods: A total of 35 healthy participants randomly conducted either exercises targeting medial thigh muscles (intervention group) or exercises that did not target specific lower extremity muscles (control group). Three-dimensional hip, knee, and ankle kinematic/kinetic data and muscle activity for lower extremity muscles were obtained during single-leg landing. Two-way analyses of variance were conducted for each dependent variable to determine the effect of 8-week of strength training targeting medial thigh muscles. Results: The intervention group showed decreased knee varus-valgus excursion (P = 0.009), peak valgus moment (P = 0.032), and peak hip internal rotation moment (P = 0.009) but increased gluteus medius activity in the precontact phase (P = 0.012) and vastus medialis-to-vastus lateralis (VM: VL) coactivation ratio in the postcontact phase (P = 0.043). The change in coronal plane knee excursion was negatively correlated with both the change in gluteus medius activity (R² = 0.321, P = 0.014) and the change in VM: VL coactivation ratio (R² = 0.276, P = 0.025). Conclusions: Strength training targeting medial thigh muscles can modify the biomechanics associated with ACL injuries; thus, this intervention might be considered when designing ACL injury prevention programs to reduce dynamic knee valgus during sports-related tasks.

Massive irreparable rotator cuff tears (RCTs) affect the clinical outcomes of reverse shoulder arthroplasty (RSA). However, the effects of subscapularis repair on the outcomes of RSA, based on the degree of posterior-superior RCTs, are unclear. This study aimed to examine the effect of subscapularis repair on three-dimensional joint contact forces (JCFs) based on the degree of posterior-superior RCT severity in RSA. Ten human in vivo experimental data were used as input to the musculoskeletal model. A six-degrees-of-freedom (DOF) anatomical shoulder model was developed and validated against three-dimensional JCFs. The 6-DOF musculoskeletal shoulder model of RSA was then developed by importing the reverse shoulder implant into the validated anatomical shoulder model. Based on the various types of posterior-superior RCT severity, inverse dynamic simulations of subscapularis-torn and subscapularis-repaired models of RSA were performed: from isolated supraspinatus tears to partial or massive tears of the infraspinatus and teres minor. The intact rotator cuff model of RSA was also simulated for comparison with the different types of models. Our results showed that the more posterior-superior RCTs progressed in RSA, the more superior JCFs were observed at 90°, 105°, and 120° abduction in the subscapularis-torn model. However, subscapularis repair decreased the superior JCF at those angles sufficiently. In addition, the teres minor muscle-tendon force increased as infraspinatus bundle tears progressed in both the subscapularis-torn and -repaired models, in order to compensate for the reduced force during abduction. However, the teres minor muscle-tendon force was not as high as that of the infraspinatus muscle-tendon, which could result in muscle force imbalance between repaired subscapularis and teres minor. Therefore, our results suggest that repairing the subscapularis and the repairable infraspinatus during RSA can improve glenohumeral joint stability in the superior-inferior direction by restoring muscle force balance between the anterior cuff (i.e., subscapularis) and posterior cuff (i.e., infraspinatus and teres minor). The findings of this study can help clinician decide whether to repair the rotator cuff during RSA to enhance joint stability.

The purpose of this study was to investigate the relationship between muscular parameters of quadriceps/hamstrings and knee joint kinetics in gait. Muscle architecture (thickness, pennation angle, and fascicle length), and quality (echo intensity) of individual quadriceps and hamstrings of 30 healthy participants (16 males and 14 females) was measured using ultrasound. Peak knee flexion moment (KFM), KFM impulse, peak knee adduction moment (KAM), and KAM impulse during walking were obtained at preferred speed. Pearson’s correlation coefficient and multiple regression analyses were performed at significance level of 0.05, and Cohen’s f² values were calculated to examine the effect sizes of multiple regression. The hamstring-to-quadriceps muscle thickness ratio (r = 0.373) and semitendinosus echo intensity (r = − 0.371) were predictors of first peak KFM (R² = 0.294, P = 0.009, f² = 0.42), whereas only vastus medialis (VM) echo intensity was a significant predictor of second peak KFM (r = 0.517, R² = 0.267, P = 0.003, f² = 0.36). Only the VM thickness was the predictor of first (r = 0.504, R² = 0.254, P = 0.005, f² = 0.34) and second peak KAM (r = 0.581, R² = 0.337, P = 0.001, f² = 0.51), and KAM impulse (r = 0.693, R² = 0.480, P < 0.001, f² = 0.92). In conclusion, the greater hamstring-to-quadriceps muscle thickness ratio and the muscle architecture and quality of medial quadriceps/hamstring play an important role in KFM and KAM, and may have implications in knee osteoarthritis.

Although studies on terrain identification algorithms to control walking assistive devices have been conducted using sensor fusion, studies on transition classification using only electromyography (EMG) signals have yet to be conducted. Therefore, this study was to suggest an identification algorithm for transitions between walking environments based on the entire EMG signals of selected lower extremity muscles using a deep learning approach. The muscle activations of the rectus femoris, vastus medialis and lateralis, semitendinosus, biceps femoris, tibialis anterior, soleus, medial and lateral gastrocnemius, flexor hallucis longus, and extensor digitorum longus of 27 subjects were measured while walking on flat ground, upstairs, downstairs, uphill, and downhill and transitioning between these walking surfaces. An artificial neural network (ANN) was used to construct the model, taking the entire EMG profile during the stance phase as input, to identify transitions between walking environments. The results show that transitioning between walking environments, including continuously walking on a current terrain, was successfully classified with high accuracy of 95.4 % when using all muscle activations. When using a combination of muscle activations of the knee extensor, ankle extensor, and metatarsophalangeal flexor group as classifying parameters, the classification accuracy was 90.9 %. In conclusion, transitioning between gait environments could be identified with high accuracy with the ANN model using only EMG signals measured during the stance phase.

Background: Muscle thickness can influence the joint kinematics and/or kinetics during dynamic activities. The relationship between the muscle thickness of individual quadriceps and hamstrings or medial-to-lateral thigh muscle thickness ratio and the knee kinematics/kinetics with respect to anterior cruciate ligament (ACL) injury risk remains unclear. Hypothesis: Higher medial-to-lateral thigh muscle thickness ratio would be associated with lower knee valgus angle/moment and lower tibial internal rotation angle/moment during single-leg landing. Study Design: Cross-sectional. Level of Evidence: Level 4. Methods: Muscle thickness of the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and semitendinosus (ST) of 30 healthy participants (16 males and 14 females) were measured using ultrasound. Knee joint kinematics and kinetics during single-leg landing were obtained. Stepwise multiple regression analysis, a follow-up Fisher’s r to z test to examine the sex as a moderator, and independent t tests to evaluate sex difference were performed. Results: Both knee valgus moment ( R² = 0.466, P < 0.001) and tibial external rotation moment ( R² = 0.330, P < 0.001) at peak anterior tibial shear force during single-leg landing were negatively correlated with medial-to-lateral (ie, (VM+ST):(VL+BF)) thickness ratio regardless of sex, whereas medial-to-lateral thigh muscle thickness ratio was not correlated with knee valgus and tibial external rotation angles. Male participants exhibited higher (VM+ST):(VL+BF) thickness ratio than female participants ( P = 0.005), and lower knee valgus moment ( P = 0.04) and tibial external rotation moment ( P = 0.05), as well. Conclusion: The knee joint moments in frontal and transverse planes during single-leg landing were associated with the medial-to-lateral thigh muscle thickness ratio; thus, the medial-lateral thigh muscle thickness could be a potential contributor to frontal and transverse plane knee joint loading during dynamic movement. Clinical Relevance: Strength training that aims to selectively strengthen the medial/lateral thigh muscles might be considered in a new ACL injury prevention training program to alter the biomechanical parameters associated with ACL injuries.

Ankle landing strategies affects the biomechanical characteristics of the knee joint, especially knee frontal plane loading. However, no studies have investigated whether the association between ankle landing biomechanics in sagittal plane and the knee frontal plane loading differs between sexes. The purpose of this study was to examine whether there is a sex difference in the effect of ankle plantar flexion at the contact angle, ankle range of motion (ROM), and ankle plantar flexion moment on knee valgus loading during single-leg landing. Twenty-five females and twenty-four males performed a single-leg landing. Joint kinematics and kinetics of the lower extremities were measured. The relationship between ankle biomechanics in the sagittal plane (ankle plantar flexion angle at contact, ROM, and peak ankle plantar flexion moment) and peak knee valgus moment were analyzed. In males, the larger ankle plantarflexion angle at contact and ROM were significantly associated with lower peak knee valgus moment. In addition, in males only, a greater peak ankle plantar flexion moment was significantly associated with a lower peak knee valgus moment and greater peak ankle inversion moment. Altering ankle landing strategies in the sagittal plane during single-leg landing may reduce the knee valgus moment, which is one of risk factors for anterior cruciate ligament injury, in males only.

The purpose of this study was to examine whether there is a sex difference in the effect of ankle plantar flexion at the contact angle, ankle range of motion (ROM), and ankle plantar flexion moment on knee valgus loading during single-leg landing. Twenty-five females and twenty-four males performed a single-leg landing. Joint kinematics and kinetics of the lower extremities were measured. Correlation coefficients were used to assess the relationship between ankle biomechanics in the sagittal plane (ankle plantar flexion angle at contact, ROM, and peak ankle plantar flexion moment) and peak knee valgus moment. In males, the larger ankle plantarflexion angle at contact and ROM were significantly associated with lower peak knee valgus moment (r = − 0.46, p < 0.05; and r = − 0.42, p < 0.05). In addition, in males only, a greater peak ankle plantar flexion moment was significantly associated with a lower peak knee valgus moment (r = − 0.83, p < 0.001) and greater peak ankle inversion moment (r = 0.75, p < 0.001). Altering ankle landing strategies in the sagittal plane during single-leg landing may reduce the knee valgus moment, which is one risk factors for anterior cruciate ligament injury, in males only.

The purposes of this study were to develop a cruciate-retaining total knee arthroplasty musculoskeletal model, which enables the adjustment of ligament length and implant alignment; validate the model; and evaluate the effects of varus/valgus alignment adjustment and unbalanced medial/lateral ligament laxity during gait. A cruciate-retaining total knee arthroplasty musculoskeletal model was constructed and validated against the in vivo contact forces. This model was transformed to 2° varus/valgus alignment of femoral or tibial replacement models and 2° medial/lateral laxity models. The contact forces and ligament tensions of the adjusted models were calculated. The contact forces in the model showed good agreement with the in vivo contact forces. Valgus replacement alignment with balanced ligament models showed a lower contact force at the medial compartment than at the neutral alignment model, whereas the varus replacement alignment with balanced ligament models showed a greater contact force at the medial compartment and medial/posterior cruciate ligament tension. The medial laxity with neutral alignment model showed a similar contact force with decreased medial ligament tension compared to the balanced neutral alignment model, whereas the lateral laxity with the neutral alignment model showed a greater contact force and decreased lateral ligament tension. The cruciate-retaining total knee arthroplasty model was validated using in vivo contact forces (r = 0.939) Two degrees of valgus alignment adjustment with balanced ligament or neutral alignment with 2° of medial laxity can be safe without increasing contact force or ligament tension compared to neutral alignment with a balanced extension gap. However, 2° of varus alignment adjustment with balanced ligament or neutral alignment with 2° of lateral laxity may be unfavorable due to the overloading of the joints and knee ligaments.