Extrinsic lower limb muscle activity is often studied using simplified segmental models that overlook distinct kinematics of individual joints within the ankle joint complex and its associated articulations. This study provides a joint-level characterization of how individual muscle groups influence talocrural, subtalar, and midtarsal joint kinematics under an unweighted condition. Six fresh-frozen cadaveric specimens were mounted to a tendon force actuator system to independently actuate six functional muscle groups: tibialis anterior (TA), extensors (EXT), peroneals (PER), flexors (FLX), tibialis posterior (TP), and Achilles. Individual stepwise loading trials were performed for the five muscle groups other than the Achilles. Each muscle group was tested in 20 % force increments while the Achilles was held at a constant 10 % baseline tension. Three-dimensional rotations at the tibiotalar (TT), talofibular, tibiofibular, subtalar (ST), talonavicular (TN), and calcaneocuboid joints were measured. Parallel coordinate plots revealed distinct muscle-specific and shared joint coordination patterns, with consistent ST/TN coronal and transverse coupled motions across muscle groups. Principal component analysis showed that the first principal component strongly reflected force-dependent motion (R2 = 0.75-0.90). TA primarily drove TT dorsiflexion and ST/TN inversion and internal rotation. EXT produced ST/TN eversion and external rotation. PER and FLX contributed greater midtarsal motions in the coronal and transverse planes, consistent with common hindfoot-midfoot stabilization strategies. FLX and TP did not generate strong TT plantarflexion, likely due to the initial plantarflexed alignment and baseline Achilles activation, but TP showed strong ST/TN inversion and internal rotation. Findings highlight the influence of initial foot posture, hindfoot-midfoot coupling, and the value of joint-level analysis for understanding muscle function, refining musculoskeletal models, and evaluating surgical interventions.

The glenoid labrum is a collagen‑rich fibrocartilage structure critical to shoulder stability yet susceptible to age‑related degeneration. Quantitative, in‑vivo assessment of its composition remains limited. Ultrashort echo time MRI with magnetization transfer (UTE‑MT) enables estimation of macromolecular fraction (MMF), a biomarker sensitive to the collagen‑rich matrix content of the glenoid labrum. This study investigated the effect of age, sex, menopause status, and arm dominance on the glenoid labrum MMF. Forty‑two healthy adults (25 females, mean age: 47 ± 15 years; 17 males, mean age: 38 ± 15 years) underwent bilateral shoulder MRI on a 3T system using a double echo steady state sequence (DESS) for segmentation and a custom UTE‑MT protocol to quantify MMF. Labra were segmented from DESS images, and voxel‑wise MMF was computed. Mean MMF was calculated for each arm. A linear mixed-effects model with a random intercept for subject to account for within-subject dependence tested the effects of age, sex, arm dominance, and their respective interactions. A linear mixed effect model with a random intercept for subject tested the effect of menopause status among females on mean MMF. Males exhibited higher mean labral MMF than females (male: 40.4 ± 7.3%, female: 39.6 ± 7.2%; p = 0.016). Age showed no main effect on mean MMF. However, a significant sex‑by‑age interaction indicated a steeper age‑related decline in males (p = 0.0113). There was no significant effect of arm dominance on mean MMF (dominant: 41.1 ± 7.6%, non-dominant: 38.8 ± 6.9%; p = 0.412), and there was no significance found for the interactions of dominance and sex, dominance and age, or dominance, sex, and age. Among females, mean MMF did not differ by menopause status (premenopausal 39.4 ± 7.4% vs. postmenopausal 39.7 ± 7.1%; p = 0.761). UTE‑MT MRI detects sex‑specific differences in glenoid labrum MMF, with a significant sex‑by‑age interaction despite no main effect of age. These findings support MMF’s potential as a non‑invasive quantitative marker of labral composition and motivates longitudinal studies to define clinically meaningful thresholds and regional variation.

BACKGROUND:Glenoid vault defects are often observed in revision shoulder arthroplasty, and improper management of glenoid bone loss during revision to reverse total shoulder arthroplasty (rTSA) increases the risk of complications. However, there is limited understanding of the types of glenoid vault defects that occur in patients. Therefore, the purpose of this study was to three-dimensionally characterize the underlying glenoid morphology of patients with failed shoulder arthroplasties subsequently undergoing revision arthroplasty to rTSA. METHODS:Computed tomography scans of 100 patients, preoperative to revision arthroplasty to rTSA (revision group) and 35 healthy cadaveric control scapulae (control group), were three-dimensionally reconstructed. A negative projection glenoid vault was created and residual screw or peg holes were removed and smoothed. Glenoid defects were quantified using various size, area, volume, and geometric measures of the negative projection and the glenoid. A cluster analysis was performed on the revision group to group patients based on glenoid shape and find patterns in the underlying glenoid morphology using principal measures: normalized distance (height; width; maximum projection thickness; and smoothed maximum residual vault thickness), angular (anterior tilt; inclination; and version), and radius of curvature (ROC) (superior-inferior ROC; anterior-posterior ROC; and vault-sphere ROC). RESULTS:When compared to the control group, the measures of the revision group showed more variation with higher standard deviations. The cluster analysis formed 2 clusters, between which 6 of the principal measures had statistically significant differences. Of the principal measures, cluster A had larger height and maximum projection thickness, more retroversion, and smaller ROCs when compared to cluster B. Of the other measurements, projection volume, lateral surface area, circumferential surface area, and height-width ratio were larger in cluster A than in cluster B. CONCLUSIONS:Glenoid morphology after failed arthroplasty is highly variable and may require novel defect management strategies. Additionally, the formation of clusters based on glenoid morphology indicates that patterns exist in the types of glenoid defects, highlighting a need to further investigate a three-dimensional classification system and potentially new standardized revision implant component designs for revision to rTSA cases.

Background:The reverse shoulder arthroplasty (RSA) angle quantifies inclination of the inferior glenoid and, thus the correction required during reverse total shoulder arthroplasty. There are no data on the effect of two-dimensional (2D) radiographic projection on the RSA angle or on the relationships between 2D true anterior-posterior (AP) views and their three-dimensional (3D) counterparts. Methods:Digitally reconstructed radiographs of the 2D true AP view of N = 68 scapulae, a subset in controlled ante-/retroversion and extension/flexion views, and their corresponding 3D anatomic models were analyzed. The RSA angle was measured on true AP images with the glenoid in profile at the intersection of lines defined by the supraspinatus fossa and inferior glenoid rim. On altered viewing perspectives, the glenoid face was visible, and thus the anterior and posterior rims. Since it was often unclear which was anterior and posterior, for consistency the RSA angle was measured at the most medial and lateral rims, and glenoid midpoint to determine the influence of measurement location. The 3D RSA angle was measured on 3D models using semi-automated techniques. Data were analyzed to determine the effects of viewing perspective and measurement location on the RSA angle and to compare 2D true AP to 3D measures. Results:The 2D RSA angle was 18.1 ± 7.1° (range: 1.1° to 35.3°), while the 3D RSA angle was 10.1 ± 7.3° (-8.1° to 25.7°) (P < .001). Ante-/retroversion views had large effects on the RSA angle. The lateral rim was the most susceptible to error (up to 25.8 ± 6.6°) and the glenoid midpoint was least susceptible (less than 5.6 ± 6.5°). Extension/flexion was also influential, but the magnitudes were generally much less than ante/retroversion. Trends due to viewing perspective differed between the medial rim, lateral rim, and glenoid midpoint. The glenoid midpoint maintained the flattest distribution with the smallest errors across the views. Inter- and intra-rater reliability in measuring RSA angles was good to excellent (≥ 0.754). Conclusion:The 2D RSA angle experiences viewing perspective errors when not measured on a true AP radiograph. The glenoid midpoint provided the most consistent and smallest maximum error, with good to excellent reliability. The 3D underestimated the 2D RSA angle on true AP images by an average of -8.0°, with similar variability. When using 2D imaging, a true AP image is desirable; otherwise, use the glenoid midpoint to minimize viewing perspective errors. Comparisons between radiographic measures and 3D preoperative planning should consider the bias between the two techniques, and 3D measures should be evaluated for their measurement techniques within the respective preoperative planning softwares to ensure consistency and reliability among manufacturers. Level of evidence:Basic Science Study; Cadaveric Study.

Muscle lengths and moment arms are necessary to quantify musculoskeletal joint torques and subsequent rotations created via muscle contractions. High-precision morphologic and kinematic data collected with dynamic stereoradiography allow semi-automatic calculation of muscle lengths and moment arms using computational methods coded into FEBio Studio. A verification/validation model of known geometry was created, simulating glenohumeral anatomy and motion, where anatomic landmarks and muscle lines of action vectors were used to calculate muscle length and moment arms dynamically throughout a motion. Moment arms were also decomposed about anatomic coordinate systems to describe the muscle’s relative contributions to physiologic planes of motion. A use case of pre- and post-operative reverse total shoulder arthroplasty was then demonstrated, where resultant changes in muscle lengths and moment arms due to the non-anatomic joint replacement agreed with prior studies. These methods, now integrated into FEBio Studio, will enable researchers and clinicians to study the effects of changing morphology and kinematics on dynamic musculoskeletal function related to muscle length and moment arms in an open-source platform.

BACKGROUND:Scapular dyskinesis is frequently observed with various types of shoulder instability, but whether scapular dyskinesis could contribute to shoulder instability is still unclear. The purpose of this study was to determine the effects of scapular orientation on anterior and posterior glenohumeral translation using a cadaveric model of anterior and posterior labral tears. METHODS:Twenty fresh-frozen cadaveric shoulders were divided into 2 groups: the anterior lesion (n = 10) and posterior lesion (n = 10) groups. The humeral head was translated anteriorly or posteriorly with a constant 30 N force in the anterior or posterior tear groups, respectively. Humeral head displacement was measured at neutral scapula orientation for the intact labrum and following anterior or posterior labral tears. Following a labral tear, humeral head displacement was also measured at 6 additional scapular orientations (±10° increments from neutral), including downward rotation, upward rotation, posterior tilt, anterior tilt, internal rotation, and external rotation. The humerus was held at 0° of horizontal abduction and 40° of horizontal abduction (the apprehension test position) or 40° of horizontal adduction (the jerk test position) in the anterior lesion or posterior lesion groups, respectively. RESULTS:The presence of isolated labral tears generally increased anterior and posterior translations on the order of 1-2 mm in the neutral scapular orientation (P ≤ .021). Anterior humeral head translation in 0° humeral abduction further increased by approximately 1 mm in the mean upward scapular rotation orientation (P ≤ .021). In the apprehension test, anterior translation increased from posterior to anterior scapular tilt (1.3 mm, P = .017), and from internal to external scapular rotation (1.8 mm, P ≤ .006). Posterior humeral translation in 0° humeral abduction showed trends increasing from downward to upward scapular rotation (1.2 mm, P ≤ .027) and posterior to anterior scapular tilt (2.8 mm, P ≤ .007), while slightly decreasing from internal to external scapular rotation (0.6 mm, P = .014). Posterior translation in the jerk test increased from downward to upward scapular rotation (0.8 mm, P ≤ .012) and posterior to anterior scapular tilt (0.9 mm, P ≤ .043), but slightly decreased from internal to external scapular rotation (0.6 mm, P = .001). CONCLUSION:Increased scapular upward rotation, anterior tilt, and external rotation were associated with increased anterior translation of the humeral head in shoulders with anterior labral lesions. In shoulders with posterior labral lesions, increased scapular upward rotation, anterior tilt, and internal rotation were associated with increased posterior translation of the humeral head. These findings suggest that scapular dyskinesis could contribute to instability recurrence.

BACKGROUND:Lesser tuberosity osteotomy (LTO) repair in total stemless shoulder arthroplasty has a high nonunion rate and thus presents a challenge to shoulder surgeons. Improved repair techniques may mitigate nonunion from excessive fragment motion or inadequate compression. The purpose of this study was to evaluate the biomechanical properties of 3 LTO repair techniques in the setting of stemless shoulder arthroplasty. Tensionable cortical button (Button) and suture anchor (Anchor) constructs were compared to a reference suture with a lateral plate construct (Suture). METHODS:Forty human cadaveric shoulders from 20 matched pairs were dissected and the lesser tuberosity osteotomized. In each pair, the LTO was repaired with a Suture and lateral plate construct on one side and on the other side, either a tensionable cortical Button or suture Anchor construct was used. Sutures were passed through the humeral components of a stemless arthroplasty system during the procedures. All specimens were cycled 1,000 times from 10 to 100 N at 1 Hz using a custom subscapularis cryoclamp. Cyclic construct gapping was recorded at regular intervals with a digital video system. All specimens were then loaded to failure, and failure load, displacement, mode, and construct stiffness were recorded. Statistical analyses compared the Suture constructs to their paired Button or Anchor constructs. RESULTS:Compared to their Suture pairs, the Button group displayed no differences in construct gapping (P ≥ .138), but the Anchor group displayed up to 50% increased gapping from cycles 1-400 (P ≤ .049). The Suture construct supported approximately 25% higher loads prior to failure when compared to their paired Button and Anchor group (P ≤ .014). There were no differences in failure stiffness between the Sutures and their paired Buttons or Anchors. CONCLUSION:In this controlled laboratory study, the decreased rate of initial construct gap formation and greater failure load of the suture construct suggests an environment for superior in vivo healing of the LTO as a result of decreased micromotion, with clinical implications still to be determined. Furthermore, the greater failure load in the suture construct could prevent catastrophic failure of the LTO in the delicate postoperative period. In the setting of total shoulder arthroplasty, an LTO subscapularis repair using a suture with a lateral plate construct provides a biomechanically superior repair to either a tensionable cortical button or suture anchor repair.

BACKGROUND:Accurate implant placement is desirable in reverse total shoulder arthroplasty (rTSA). Prior studies have focused on the accuracy of patient-specific glenoid implant placement, with little data on humeral preparation. This study quantified both glenoid and humerus implant placement, with specific focus on differences between inlay and onlay humeral implant systems. METHODS:Twenty cadaver shoulders were preoperatively planned in Blueprint software. After the rTSA was placed, computed tomography scans were obtained, and 3-dimensional models of the bones and implants were created. Errors were assessed for factors such as glenoid guide pin location, reaming depth, inclination, and version, as well as humeral implant height, inclination, and version, by comparing the models from planned and performed surgeries. RESULTS:Glenoid guide pin placement was accurate, with an error of mean ± standard deviation of 1.6 ± 0.7 mm. Although glenoid reaming depth appeared accurate (0.9 ± 1.4 mm), the variability between specimens ranged from -2.5 mm of over-reaming to 4.8 mm of under-reaming. Consequent glenoid inclination and version showed small mean error in inlay and onlay systems (1.4° ± 3.9° and -1.4° ± 4.2°, respectively) but large ranges among specimens (15.8° and 20.0°, respectively). Likewise, humeral implant height relative to the bone had small mean error (0.5 ± 3.4 and -0.8 ± 1.1 mm, respectively). The inlay height variation was nearly 3 times that of the onlay (11.8 and 3.6 mm, respectively). Error in effective humeral inclination was again small (0.9° ± 3.3° and -1.8° ± 3.7°, respectively), with large overall variation (11.4° and 12.4°, respectively). Humeral implant version was the most error-prone, where specimen-specific variation ranged nearly 40° between the plan and procedure. The glenoid or humeral preparation error did not differ between the inlay and onlay systems for any metric (P ≥ .059). CONCLUSIONS:This study quantified the error in placing glenoid and humeral implants relative to a preoperative plan for both humeral inlay and onlay systems. With the exception of humeral implant version, the mean placement error was generally small. In contrast, specimen-specific implant placement had large errors, even when the average appeared acceptable. Device manufacturers should evaluate the ability of their preoperative planning software (like Blueprint) and instrumentation to control these variables in practice, especially humeral implant version, and specifically account for anatomic humeral torsion as it likely has a substantial impact on humeral implant placement. These procedure-to-procedure variations have biomechanical implications that could affect shoulder function and complications and negatively impact outcomes of patient-specific rTSA if left unchecked.

Background: Reverse total shoulder arthroplasty is a common procedure for end-stage glenohumeral arthritis, rotator cuff disease, fracture, or failed arthroplasty. Preoperative planning software allows surgeons to assess implant placement and relies on imported bone models that come from imaging, typically a supine computed tomography (CT) scan. However, daily activities are performed upright. This study quantified the differences in scapulothoracic and glenohumeral joint angles between preoperative supine and upright imaging. Methods: Seven patients underwent preoperative supine CT and upright biplane fluoroscopy imaging. Scapulothoracic and glenohumeral joint angles in supine and upright poses were calculated using Euler angles and compared using 2-sided paired t-tests. Results: The scapula was 9.3 ± 12.9° more downward rotated in the upright pose than supine (P = .004), while the humerus was 13.2 ± 17.6° more elevated (P = .020). Patient-specific changes varied widely for scapulothoracic upward/downward rotation (+0.5° to −17.6°), pro/retraction (+10.4° to −22.4°), and posterior/anterior tilt (+17.4° to −9.3°), as well as glenohumeral elevation/depression (+26.7° to −4.0°), anterior/posterior plane of elevation (+19.3° to −5.6°), and internal/external axial rotation (+5.5° to −44.6°). Conclusion: Patient-specific variation in joint angles between supine and upright poses demonstrates gross changes in both magnitude and direction that exceed common thresholds in implant selection and placement. Thus, preoperative planning using supine CT may inaccurately pose bones, with consequent effects on the surgical plan, the resultant shoulder biomechanics, and clinical outcomes. Preoperative planning software should consider the influence of supine posture when orienting bones for surgical planning.

Background: Ulnar collateral ligament (UCL) tears are a common injury among overhead athletes, leading to a lengthy recovery period after surgical reconstruction. To explore alternatives to decrease recovery time, we compared the biomechanical properties of the standard palmaris reconstruction with a novel bone-patellar tendon-bone (BTB) reconstruction with interference screw fixation. Methods: Sixteen fresh frozen cadaver upper extremities and 8 cadaver knees were obtained (elbows 77 ± 9 years, male:female 1:3 ratio; knees 59 ± 7 years, male:female 3:1 ratio). Eight upper extremities were randomized to the BTB group and 8 to the palmaris longus group. The elbow was placed in neutral rotation at 90° of flexion and affixed to a custom test frame. A motion capture system was used to track displacement of the humeral construct insertion relative to the ulnar construct insertion. Loading was applied to the forearm to first induce a 0.5 Hz cyclic torque at the joint between 0.5 and 5 Nm for 500 cycles. Load to failure was then performed until either ligament failure or maximum displacement of the actuator was reached. This loading protocol was first performed for the native UCL, then the ligament was reconstructed with either BTB or palmaris longus, then the protocol was repeated. Results: The native UCL of the cadavers did not differ between the groups in initial ligament length, elongation during cycling, stiffness, or opening angle at failure. The palmaris reconstruction did not differ from the native UCL for any metric. The palmaris reconstruction demonstrated a higher torque at failure when compared to the BTB reconstruction (13.7 ± 4.9 Nm vs. 5.9 ± 1.0 Nm; P = .001). A majority of the palmaris reconstructions (5/8) failed via graft rupture. Of the BTB reconstructions, 7 of 8 failed during cycling. Of these, 6 of 8 demonstrated a humeral-sided graft rupture at the tendon bone interface whereas the other 2 failed on the ulnar-sided graft at the tendon bone interface. Conclusion: The UCL reconstruction using a BTB fixated with interference screws demonstrated a high failure rate, with the graft largely failing at the bone–tendon junction on the humeral side. Standard practice UCL reconstruction with palmaris longus did not differ from the native UCL, and thus should remain the preferred graft choice among these 2 options.

Background: Recently, all-suture, all-inside meniscal repair devices—including devices containing flat sutures or tapes—have been introduced. Similar to those in suture anchors, these modifications may have different performance characteristics than conventional sutures and polyether ether ketone (PEEK)-anchored devices. Purpose: To compare the biomechanical characteristics of all-suture meniscal repair devices with those of a conventional PEEK-anchored device and an inside-out meniscal suture construct. Study Design: Controlled laboratory study. Methods: A total of 48 adult porcine menisci with simulated bucket-handle tears were included. Single-device repairs were performed with the SuperBall Meniscal Repair System, FiberStitch, and FAST-FIX 360 with 2 PEEK anchors, and a vertical mattress inside-out suture repair was performed using a Ti-Cron No. 2-0 braided polyethylene terephthalate suture. All specimens were preloaded (10 N) and cycled 200 times (between 10 and 50 N). Specimens surviving cyclic loading were then destructively tested. Endpoints included maximum failure load, stiffness, cyclic displacement, and failure mode. The goal was 12 successful tests in each group. Metrics between groups were compared using analysis of variance with post hoc tests to control for multiple comparisons. Results: The SuperBall (108.9 N) was significantly stronger than the FAST-FIX 360 (67.3 N) and Ti-Cron (75.2 N), and the FiberStitch (102.8 N) was significantly stronger than the FAST-FIX 360 ( P≤ .01 for all). Cyclic stiffness increased during cyclic loading for all constructs ( P < .001). The Ti-Cron was significantly stiffer than the SuperBall during 5 to 200 cycles ( P < .001). Cyclic displacement significantly increased in all constructs during cycling ( P < .001) but did not differ between devices. Failure mode varied by device: the Ti-Cron repairs failed because of suture breakage, the SuperBall and FAST-FIX 360 failed at the anchor, and the FiberStitch showed both failure modes. Conclusion: The all-suture, all-inside meniscal repair devices demonstrated superior strength to the PEEK-anchored device and the classic inside-out suture meniscal repair but no statistically significant difference in cyclic displacement.

The biological factors that affect healing after rotator cuff repair (RCR) are not well understood. Genetic variants in the extracellular matrix protein Tenascin C (TNC) are associated with impaired tendon healing and it is expressed in rotator cuff tendon tissue after injury, suggesting it may have a role in the repair process. The purpose of the current study was to determine the role of TNC on tendon healing after RCR in a murine model. The supraspinatus tendon was transected and repaired on the left shoulder of wild-type (WT-RCR), Tenascin C null (Tnc--RCR) and Tnc heterozygous (Tnc+/--RCR) mice. Controls included the unoperated, contralateral shoulder of WT-RCR, Tnc-RCR, Tnc+/--RCR mice and unoperated shoulders from age and genotype matched controls. We performed histologic, activity testing, bulk RNA-seq, and biomechanical analyses. At 8-weeks post-RCR, Tnc- and Tnc+/- mice had severe bone and tendon defects following RCR. Tnc--RCR mice had reduced activity after RCR including reduced wheel rotations, wheel duration, and wheel episode average velocity compared with WT-RCR. Loss of Tnc following RCR altered gene expression in the shoulder, including upregulation of sex hormone and WNT pathways and a downregulation of inflammation and cell cycle pathways. Tnc- mice had similar biomechanical properties after repair as WT. Further research is required to evaluate tissue specific alterations of Tnc, the interactions of Tnc and sex hormone and inflammation pathways as well as possible adjuvants to improve enthesis healing in the setting of reduced TNC function.

Failure of healing after rotator cuff repair (RCR) is common. The purpose of the current study was to evaluate the effect of systemic estrogen or testosterone supplementation on tendon healing after RCR. Seventy-two adult male mice were utilized for all experiments. The supraspinatus tendon was transected and repaired with 6-0 Prolene suture on the left shoulder of 51 animals. Mice were segregated into three groups postoperative: (1) vehicle group (VG; n = 18), (2) estrogen group (EST; n = 17), and (3) testosterone group (TST; n = 16). An unrepaired control group (unrepaired, n = 21) did not have surgery. Utilizing these animals, histological analysis, activity testing, biomechanical testing and RNA sequencing (RNA-seq) was performed. At 8 weeks post-RCR, TST, and EST supplementation improved the overall histologic structure of the repaired enthesis site. No differences in ultimate failure loads or stiffness were detected between VG, EST, and TST groups after biomechanical testing. RCR caused a reduction in wheel activity compared to unrepaired controls and supplementation with TST restored wheel activity. RNA-seq analysis indicated that estrogen and testosterone regulated different pathways associated with enthesis healing, including a suppression of inflammatory signaling. Supplementation with sex hormones improved the structure of the repaired tendon enthesis and significantly regulated expression of diverse pathways regulating multiple biological processes. Testosterone administration following RCR restored wheel activity without having a detrimental impact on biomechanical strength. Future human studies of sex hormone supplementation after RCR are warranted as supplementation in an animal model may improve tendon enthesis healing.

The objective of this study was to define targeted reaching performance without visual information for transhumeral (TH) prosthesis users, establishing baseline information about extended physiological proprioception (EPP) in this population. Subjects completed a seated proprioceptive targeting task under simultaneous motion capture, using their prosthesis and intact limb. Eight male subjects, median age of 58 years (range 29-77 years), were selected from an ongoing screening study to participate. Five subjects had a left-side TH amputation, and three a right-side TH amputation. Median time since amputation was 9 years (range 3-54 years). Four subjects used a body-powered prosthetic hook, three a myoelectric hand, and one a myoelectric hook. The outcome measures were precision and accuracy, motion of the targeting hand, and joint angular displacement. Subjects demonstrated better precision when targeting with their intact limb compared to targeting with their prosthesis, 1.9 cm2 (0.8-3.0) v. 7.1 cm2 (1.3-12.8), respectively, p = 0.008. Subjects achieved a more direct reach path ratio when targeting with the intact limb compared to with the prosthesis, 1.2 (1.1-1.3) v. 1.3 (1.3-1.4), respectively, p = 0.039 The acceleration, deceleration, and corrective phase durations were consistent between conditions. Trunk angular displacement increased in flexion, lateral flexion, and axial rotation while shoulder flexion decreased when subjects targeted with their prosthesis compared to the intact limb. The differences in targeting precision, reach patio ratio, and joint angular displacements while completing the targeting task indicate diminished EPP. These findings establish baseline information about EPP in TH prosthesis users for comparison as novel prosthesis suspension systems become more available to be tested.

Background: Snapping scapula syndrome (SSS) can result in crepitus and painful scapulae during motion and may be treated with bursectomy and/or superomedial angle resection. The medial scapula corpus angle (MSCA) measures blade curvature on a transverse plane below the suprascapular fossa and may indicate SSS, yet a large overlapping range in MSCA exists between patients with and without SSS. This study quantified the effects of 3-dimensional scapula orientation in the imaging field, and the resulting variability in scapula type and MSCA. Methods: Computed tomography scans from 10 healthy controls (non-SSS) and 8 SSS patients were used to create 3-dimensional scapula models. The scapula type and MSCA were measured on a controlled reference imaging plane, and ones translated and rotated below the supraspinatus fossa to create 19 planes simulating variations due to scapulothoracic orientation. Planes translated and rotated above the reference plane also generated 13 modified MSCA planes to test areas modified during surgical resection. Statistical analyses compared the scapula type and MSCA between the reference and alternate planes within groups. Results: Scapula type commonly changed and the MSCA varied up to 104° within a subject depending on the imaging plane, regardless of location below or above the reference plane. Numerous statistical differences were detected in MSCA between the reference plane and those translated and rotated below that plane in both non-SSS and SSS groups. Planes translated above the reference plane showed consistent statistical differences in MSCA to the reference plane, but only in the SSS group. Discussion: Although scapula type and MSCA were previously shown to differentiate patients, the effect of viewing perspective was not considered. Differences in scapula orientation relative to the imaging plane dramatically varied the scapula type and MSCA, far exceeding differences between groups described previously. Herein, scapula type and MSCA often differed in planes translated above the reference plane, suggesting that scapular abnormalities contributing to SSS are largely at or close to the superomedial angle. Conclusion: The MSCA as defined previously likely lacks the sensitivity and specificity to reliably be used as a clinical diagnostic tool for SSS. The blade showed consistent differences when translated above the reference plane; however, it was still highly variable. Sensitivity and specificity of planes above the reference plane should be investigated further as they may provide reliable differentiation of non-SSS and SSS patients.

Background: One method to augment rotator cuff repair is to pass dermal allograft pledgets along the sutures that bridge from the medial to the lateral row. It remains unclear whether this augmentation method alters repair biomechanics. Methods: This was a controlled laboratory study. After an a priori power analysis, 9 pairs of rotator cuffs underwent double-row suture bridge rotator cuff repair, half randomized to augmentation with dermal allograft pledgets passed along the suture bridge sutures. Repairs were then mounted on a material testing system and loaded cyclically 500 cycles to measure applied force and displacement. Repairs then underwent ultimate failure testing, and stiffness, ultimate failure force, and ultimate failure displacement were measured. Paired t tests were performed to compare between groups. Results: There were no differences between groups in construct gapping with cyclic loading after 500 cycles ( P = .885). There were no differences between the augmented and control groups in yield force (103.5 + 5.0 vs. 101.4 + 5.9 N, respectively, P = .183), stiffness (94.2 + 13.9 vs. 90.9 + 13.8, P = .585), or ultimate failure force (255.3 + 65.8 vs. 285.3 + 83.2, P = .315). There were no differences between groups in failure modes, with most specimens failing by cuff tissue tearing within or medial to the construct. Conclusion: The addition of dermal allograft pledgets does not positively or negatively influence the time -zero biomechanical characteristics of double-row suture bridge rotator cuff repair. Level of evidence: Basic Science Study; Biomechanics (c) 2024 Published by Elsevier Inc. on behalf of Journal of Shoulder and Elbow Surgery Board of Trustees.

BACKGROUND:Humeral distalization is inherent to reverse total shoulder arthroplasty (rTSA) and is often produced with concomitant humeral lateralization via the level of the humeral head cut, implant positioning, implant neck shaft angle, and polymer insert thickness. Biomechanical data on the isolated effects of humeral distalization remain limited but could be important to consider when optimizing postoperative rTSA shoulder function. This study investigated the effects of isolated humeral distalization on shoulder biomechanics using a biorobotic shoulder simulator. METHODS:Eight fresh-frozen cadaveric shoulders were tested using custom polymer inserts that translated the bearing surface 0, +5, +10, and +15 mm along the humeral stem axis, producing isolated distalization without lateralization. Specimens underwent passive elevation in the scapular plane with a static scapula to assess glenohumeral range of motion. Scapular plane abduction motion trajectories were then performed, driven by previously collected scapulothoracic and glenohumeral kinematics from rTSA patients. The effect of isolated distalization on passive elevation was tested using mixed-effects linear regression and the effect on muscle force, joint reaction force, and muscle excursion during active scapular-plane abduction was tested using statistical parametric mapping random effects analysis. RESULTS:Maximum passive scapular plane elevation increased with humeral distalization (4° per 5 mm distalization). During active elevation, deltoid and rotator cuff muscle forces, and joint reaction forces, increased up to 37% per 5 mm of distalization. Simulated deltoid muscle excursion was altered with increasing distalization but amounted to no more than 0.8 mm change from baseline per 5 mm of distalization. Rotator cuff muscles were consistently lengthened throughout abduction, up to 1.6 mm per 5 mm of distalization. These trends were observed across various patient motions. CONCLUSIONS:Isolated humeral distalization caused dramatic increases in the muscle forces required to perform scapular-plane abduction. Joint reaction forces increased correspondingly. These results suggest that implant and surgical strategies to generate deltoid muscle tension without humeral distalization may promote better active range of motion and more durable long-term outcomes over approaches that rely on distalization.

Introduction:The shoulder joint complex is prone to musculoskeletal issues, such as rotator cuff-related pain, which affect two-thirds of adults and often result in suboptimal treatment outcomes. Current musculoskeletal models used to understand shoulder biomechanics are limited by challenges in personalization, inaccuracies in predicting joint and muscle loads, and an inability to simulate anatomically accurate motions. To address these deficiencies, we developed a novel, personalized modeling framework capable of calibrating subject-specific joint centers and functional axes for the shoulder complex. Methods:We developed a novel personalized modeling framework utilizing the Joint Model Personalization (JMP) Tool from the Neuromusculoskeletal Modeling Pipeline, incorporating in vivo biplane fluoroscopy data of the glenohumeral and scapulothoracic joints. Initially, open-chain scapula-only models with 3, 4, and 5 degrees of freedom (DOFs) were created and optimized using synthetic marker data derived from subject-specific geometry. Subsequently, closed-chain shoulder models including scapula, clavicle, and humerus were constructed and optimized through a two-stage personalization approach. Model accuracy and generalizability were assessed using marker distance errors and leave-one-out cross-validation across multiple shoulder motions. Results:Increasing the number of scapula DOFs in open-chain models improved kinematic accuracy, with the 5 DOF scapula model yielding the lowest marker distance errors (average: 0.8 mm; maximum: 5.2 mm). The closed-chain shoulder model demonstrated high accuracy (average: 0.9 mm; maximum: 5.7 mm) and consistency across subject in cross-validation tests (average marker distance errors = 1.0-1.4 mm). Models personalized with synthetic noise representative of skin-based marker data resulted in slightly increased, yet acceptable marker errors (average: 3.4 mm). Conclusion:Our personalized, closed-chain shoulder modeling framework significantly improves the accuracy and anatomical fidelity of shoulder kinematic simulations compared to existing approaches. This framework minimizes errors in joint kinematics and provides a foundation for future models incorporating personalized musculature and advanced simulations.

BackgroundTranshumeral (TH) limb loss leads to loss of body mass and reduced shoulder range of motion. Despite most owning a prosthesis, prosthesis abandonment is common. The consequence of TH limb loss and prosthesis use and disuse during gait may be compensation in the upper body, contributing to back pain or injury. Understanding the impact of not wearing a TH prosthesis on upper body asymmetries and spatial-temporal aspects of gait will inform how TH prosthesis use and disuse affects the body.Research QuestionDoes TH limb loss alter upper body asymmetries and spatial-temporal parameters during gait when wearing and not wearing a prosthesis compared to able-bodied controls?MethodsEight male TH limb loss participants and eight male control participants completed three gait trials at self-selected speeds. The TH limb loss group performed trials with and without their prosthesis. Arm swing, trunk angular displacement, trunk-pelvis moment, and spatial-temporal aspects were compared using non-parametric statistical analyses.ResultsBoth TH walking conditions showed greater arm swing in the intact limb compared to the residual (p≤0.001), resulting in increased asymmetry compared to the control group (p≤0.001). Without the prosthesis, there was less trunk flexion and lateral flexion compared to the control group (p≤0.001). Maximum moments between the trunk and pelvis were higher in the TH group than the control group (p≤0.05). Spatial-temporal parameters of gait did not differ between the control group and either TH limb loss condition.SignificanceProsthesis use affects upper body kinematics and kinetics, but does not significantly impact spatial-temporal aspects of gait, suggesting these are compensatory actions. Wearing a prosthesis helps achieve more normative upper body kinematics and kinetics than not wearing a prosthesis, which may help limit back pain. These findings emphasize the importance of encouraging at least passive use of prostheses for individuals with TH limb loss.