Accuracy of free-hand humeral head resection planned on 3D-CT models in shoulder arthroplasty: an in vitro analysis.

INTRODUCTION: Three-dimensional planning of humeral head osteotomy in shoulder arthroplasty (SA) is understudied. This study evaluated whether a standard osteotomy technique along the anterosuperior anatomic neck (ASOT) could be surgically reproduced as pre-operatively planned on 3D-CT models. MATERIAL AND METHODS: Pre-operative planning in 12 cadaver shoulders was performed on a 3D-CT model of the humerus to calculate the planned osteotomy plane (planned OP). The osteotomy was then performed using a free-hand technique, and a post-operative CT scan was obtained for analysis (performed OP). Planes were compared with regards to inclination, retroversion, and resected humeral head thickness so the accuracy could be quantified. RESULTS: The absolute errors between the performed and planned OP were 2° (0-10°), 5° (0-14°), and 4 mm (1-7 mm) for inclination, retroversion, and resected head thickness, respectively. Deviation < 10° for inclination and retroversion and < 5 mm for resected humeral head thickness between planned and performed OP was achieved in 92%, 83%, 58% of cases, respectively. No differences were found for inclination (p = 0.289), whereas retroversion and resected head thickness were smaller than planned (p ≤ 0.027). CONCLUSIONS: Pre-operative planning of the ASOT using a 3D-CT model is accurate within a threshold of 10° when using a free-hand technique in 92% of cases for inclination. Retroversion and resected head thickness differed from the pre-operative plan, thereby limiting the unrestricted use of humeral head osteotomy planning from 3D-CT models in SA. These findings are a reference for further studies to develop and quantify the accuracy of pre-operative planning software including cutting guides for SA using 3D-CT models. LEVEL OF EVIDENCE: Basic science article.

In Vitro Simulation of Shoulder Motion Driven by Three-Dimensional Scapular and Humeral Kinematics.

In vitro simulation of three-dimensional (3D) shoulder motion using in vivo kinematics obtained from human subjects allows investigation of clinical conditions in the context of physiologically relevant biomechanics. Herein, we present a framework for laboratory simulation of subject-specific kinematics that combines individual 3D scapular and humeral control in cadavers. The objectives were to: (1) robotically simulate seven healthy subject-specific 3D scapulothoracic and glenohumeral kinematic trajectories in six cadavers, (2) characterize system performance using kinematic orientation accuracy and repeatability, and muscle force repeatability metrics, and (3) analyze effects of input kinematics and cadaver specimen variability. Using an industrial robot to orient the scapula range of motion (ROM), errors with repeatability of ±0.1 mm and <0.5 deg were achieved. Using a custom robot and a trajectory prediction algorithm to orient the humerus relative to the scapula, orientation accuracy for glenohumeral elevation, plane of elevation, and axial rotation of <3 deg mean absolute error (MAE) was achieved. Kinematic accuracy was not affected by varying input kinematics or cadaver specimens. Muscle forces over five repeated setups showed variability typically <33% relative to the overall simulations. Varying cadaver specimens and subject-specific human motions showed effects on muscle forces, illustrating that the system was capable of differentiating changes in forces due to input conditions. The anterior and middle deltoid, specifically, showed notable variations in patterns across the ROM that were affected by subject-specific motion. This machine provides a platform for future laboratory studies to investigate shoulder biomechanics and consider the impacts of variable input kinematics from populations of interest, as they can significantly impact study outputs and resultant conclusions.

Age-related differences in humerothoracic, scapulothoracic, and glenohumeral kinematics during elevation and rotation motions.

Age affects gross shoulder range of motion (ROM), but biomechanical changes over a lifetime are typically only characterized for the humerothoracic joint. Suitable age-related baselines for the scapulothoracic and glenohumeral contributions to humerothoracic motion are needed to advance understanding of shoulder injuries and pathology. Notably, biomechanical comparisons between younger or older populations may obscure detected differences in underlying shoulder motion. Herein, biplane fluoroscopy and skin-marker motion analysis quantified humerothoracic, scapulothoracic, and glenohumeral motion during 3 static poses (resting neutral, internal rotation to L4-L5, and internal rotation to maximum reach) and 2 dynamic activities (scapular plane abduction and external rotation in adduction). Orientations during static poses and rotations during active ROM were compared between subjects <35 years and >45 years of age (N = 10 subjects per group). Numerous age-related kinematic differences were measured, ranging 5-22°, where variations in scapular orientation and motion were consistently observed. These disparities are on par with or exceed mean clinically important differences and standard error of measurement of clinical ROM, which indicates that high resolution techniques and appropriately matched controls are required to avoid confounding results of studies that investigate shoulder kinematics. Understanding these dissimilarities will help clinicians manage expectations and treatment protocols where indications and prevalence between age groups tend to differ. Where possible, it is advised to select age-matched control cohorts when studying the kinematics of shoulder injury, pathology, or surgical/physical therapy interventions to ensure clinically important differences are not overlooked.

Reliable interpretation of scapular kinematics depends on coordinate system definition.

BACKGROUND: Interpretation of shoulder motion across studies has been complicated due to the use of numerous scapular coordinate systems in the literature. Currently, there are no simple means by which to compare scapular kinematics between coordinate system definitions when data from only one coordinate system is known. RESEARCH QUESTION: How do scapular kinematics vary based on the choice of coordinate system and can average rotation matrices be used to accurately convert kinematics between scapular local coordinate systems? METHODS: Average rotation matrices derived from anatomic landmarks of 51 cadaver scapulae (29 M/22 F; 59 ±â€¯13 yrs; 26R/25 L; 171 ±â€¯11 cm; 70 ±â€¯19 kg; 23.7 ±â€¯5.5 kg/m2) were generated between three common scapular coordinate systems. Absolute angle of rotation was used to determine if anatomical variability within the cadaver population influenced the matrices. To quantify the predictive capability to convert kinematics between the three coordinate systems, the average rotation matrices were applied to scapulothoracic motion data collected from 19 human subjects (10 M/9 F; 43 ±â€¯17 yrs; 19R; 173 ±â€¯9 cm; 71 ±â€¯16 kg; 23.6 ±â€¯4.5 kg/m2) using biplane fluoroscopy. Root mean squared error (RMSE) was used to compare kinematics from an original coordinate system to the kinematics expressed in each alternative coordinate system. RESULTS: The choice of scapular coordinate system resulted in mean differences in scapulothoracic rotation of up to 23°, with overall different shapes and/or magnitudes of the curves. A single average rotation matrix between any two coordinate systems achieved accurate conversion of scapulothoracic kinematics to within 4° of RMSE of the known solution. The average rotation matrices were independent of sex, side, decomposition sequence, and motion. SIGNIFICANCE: Scapulothoracic kinematic representations vary in shape and magnitude based solely on the choice of local coordinate system. The results of this study enhance interpretability and reproducibility in expressing scapulothoracic motion data between laboratories by providing a simple means to convert data between common coordinate systems. This is necessitated by the variety of available motion analysis techniques and their respective scapular landmark definitions.

Humeral head osteotomy in shoulder arthroplasty: a comparison between anterosuperior and inferoanterior resection techniques.

BACKGROUND: The best chance that a shoulder arthroplasty will restore motion and muscle balance across the glenohumeral joint is by closely replicating natural articular morphology. Defining the humeral osteotomy plane along clear landmarks at the anatomic neck is critical. We hypothesized that a new osteotomy, based on alternative landmarks on the anatomic neck, would restore 3-dimensional humeral head morphology more reliably than the traditional osteotomy. METHODS: The anatomic neck was digitized in 30 human cadaver shoulders and compared with its 3-dimensional computed tomography reconstruction. Two different osteotomy techniques were virtually performed: the traditional, following the anterosuperior anatomic neck; and a new technique, defined by the inferoanterior anatomic neck. The length-width difference and orientation (retroversion, inclination) of the resection area were compared between the techniques and with native anatomy. RESULTS: Length-width difference of the anterosuperior resection area was higher than in the inferoanterior osteotomy (6 ± 2 mm vs. 3 ± 1 mm; P < .001). Retroversion of the anterosuperior resection plane was higher than the native head (50° ± 12° vs. 37° ± 11°; P < .001), whereas retroversion after the inferoanterior osteotomy (32° ± 12°) did not differ from native (P = .057). Inclination differed after the anterosuperior osteotomy (129° ± 5°) and the inferoanterior osteotomy (127° ± 4°) compared with the native head (134° ± 4°; P ≤ .001). CONCLUSION: The inferoanterior referenced osteotomy generated a more circular resection area, matching the native humeral head retroversion more closely than in the anterosuperior technique. This study suggests that in shoulder arthroplasty, the humeral resection level should be referenced at the inferoanterior rather than the anterosuperior anatomic neck. Further studies should investigate the biomechanical effects of this alternative resection plane.

Biomechanics of Polyhydroxyalkanoate Mesh-Augmented Single-Row Rotator Cuff Repairs.

Polyhydroxyalkanoate (PHA) mesh is a bioresorbable scaffold used to reinforce the suture-tendon interface in rotator cuff repairs (RCRs). We conducted a study of cyclic and ultimate failure properties of PHA mesh-augmented single-row RCRs and nonaugmented RCRs. Eight pairs of fresh-frozen cadaver humeri (6 male, 2 female) were tested. Mean (SD) age was 61 (9) years. The supraspinatus tendon was resected and reattached in a single-row configuration using 2 triple-loaded suture anchors and 6 simple stitches. The opposite humerus underwent RCR augmented with 2 strips of 13-mm × 23-mm PHA mesh. Humeri were mounted in an Instron load frame, cycled 1000 times to 1.0 MPa of effective stress, and loaded to failure. Construct gapping and ultimate failure loads/displacements were recorded. Paired t tests compared augmented and nonaugmented RCRs (P ≤ .05 was significant). There was no difference in gapping over 1000 cycles (P = .879). Mean (SD) failure load was higher for PHA mesh-augmented RCRs, 571 (173) N, than for nonaugmented (control) RCRs, 472 (120) N (P = .042), and failures were consistent within pairs because of tissue failure at the knots or anchor pullout. This technique for arthroscopic augmentation can be used to improve initial biomechanical repair strength in tears at risk for failure.

Regional mechanical properties of the long head of the biceps tendon.

BACKGROUND: Long head of the biceps tenodesis reliably relieves pain, and restores strength, stability, and normal appearance of the upper extremity in the event of biceps tendinopathies. Regional differences in tendon mechanics may provide surgeons with valuable guidance in the placement of the tenodesis repair construct. The purpose of this study was to compare the mechanical properties of the long head of the biceps tendon in three functional regions of the tendon: intra-articular (proximal), suprapectoral (middle), and subpectoral (distal). METHODS: Uniaxial tensile tests were performed on the long head of the biceps tendon segments to quantify the material and structural properties of the tendon. Material properties were obtained using dogbone-shaped specimens while structural properties were obtained using intact specimens where the clamp boundary conditions simulated the common "gold standard" tenodesis, the interference screw. FINDINGS: Elastic modulus for the supra- and subpectoral regions were significantly greater than the intra-articular region (P≤0.048). The tensile strength of the subpectoral region tended to be lower compared to all other functional regions (P=0.051). The failure mechanism for intact specimens was similar to that seen for interference screw fixation where tissue failure occurs due to tearing at the bone/tendon/screw interface. INTERPRETATION: The higher tensile strength of the suprapectoral region compared to the subpectoral region may make this a more desirable location for tenodesis placement based on tissue strength. Similar elastic moduli and structural stiffness between the supra- and subpectoral regions indicate that the construct type may play a bigger role in functional outcomes in relation to construct deformation.

Suture placement near the musculotendinous junction in the supraspinatus: implications for rotator cuff repair.

BACKGROUND: Transosseous-equivalent rotator cuff repair has an increased incidence of medial rotator cuff failure compared with single-row repair. No studies have evaluated the influence of the proximity of the suture row to the musculotendinous junction (MTJ) on cyclic gapping and failure properties. HYPOTHESIS: A single row of horizontal mattress sutures placed within the supraspinatus tendon lateral to the MTJ will experience less gap formation and higher failure loads than a similar suture row placed at the MTJ. STUDY DESIGN: Controlled laboratory study. METHODS: Paired supraspinatus tendons were isolated from human cadaveric specimens and resected at the tendon insertion to the humerus. Randomized within a pair, a single row of 4 horizontal mattress sutures was placed either in the tendon 5 mm lateral to the MTJ or at the MTJ. The tied sutures secured the tendon to a fixture that ensured consistent placement of the suture row in the tendon and static fixation of the row. The muscle belly was gripped in a cryoclamp, and a servohydraulic materials testing machine was used to provide uniaxial tensile deformation for 500 cycles at 1 Hz, followed by load to failure at 1 mm/s. Fiducial markers with video tracking were used to quantify gap formation at the suture line, while the materials testing machine recorded loading for the cyclic and failure tests. RESULTS: During cyclic loading, both constructs experienced gross initial gap formation, followed by progressive gap formation that plateaued after cycle 200. The MTJ specimens had significantly higher mean cumulative gapping than the tendon specimens: 3.6±1.0 mm versus 2.4±0.6 mm, respectively (P=.012). The tendon specimens had significantly higher mean loads to failure than did the MTJ specimens: 567.1±121.8 N versus 434.2±148.1 N, respectively (P=.013). The mean failure displacement did not differ between groups for the tendon and MTJ: 5.7±2.5 mm versus 4.5±2.0 mm, respectively (P=.144). CONCLUSION: A horizontal suture row placed at the MTJ has inferior mechanical properties (increased gapping, decreased load support) as compared with a suture row placed 5 mm laterally within the tendon. CLINICAL RELEVANCE: The integrity of rotator cuff repair may be compromised if sutures are placed too close to the MTJ.

Characterization of plantaris tendon constructs for ankle ligament reconstruction.

BACKGROUND: Many techniques have been described for lateral ligament reconstruction. One frequently overlooked autograft option is the plantaris tendon, potentially due to the paucity of data on its mechanical characteristics. This study examined the structural properties of double and quadruple plantaris tendon constructs. METHODS: Plantaris tendons were harvested from 35 fresh-frozen human cadaver specimens (mean age, 66 years [range, 43-89 years]; 17 female, 13 male). The tendon ends were sutured in a running locking technique and then woven onto a template board to create double or quadruple graft constructs with a 20-mm functional length. If additional tendon length remained, a single 40-mm specimen was isolated to provide tissue material properties. Structural properties were calculated from the results of cyclic and failure uniaxial tensile tests. RESULTS: Quadruple-strand constructs had a tensile strength of 205.8 ± 68.2 N and a stiffness of 133.1 ± 46.3 N/mm. Single strands had a tensile strength of 66.9 ± 26.3 N and a stiffness of 43.8 ± 14.7 N/mm. Material properties were similar to a prior study. CONCLUSIONS: The average maximum tensile strength for the quadrupled plantaris grafts exceeded the strength of the intact anterior talofibular ligament of 139 to 161 N; therefore, the quadruple plantaris construct may be a viable autograft for foot and ankle ligament reconstruction. CLINICAL RELEVANCE: The tensile strength of the plantaris tendon is comparable to, or stronger than, other grafts already in use and offers a donor site that may result in negligible loss of strength.