A Posterior Acromial Bone Block Augmentation Is Biomechanically Effective at Restoring the Force Required To Translate the Humeral Head Posteriorly in a Cadaveric, Posterior Glenohumeral Instability Model.

PURPOSE: To assess the biomechanical utility of a posterior acromial bone block (PABB) for the treatment of posterior glenohumeral instability. METHODS: Ten fresh-frozen cadaveric specimens were obtained based upon an a priori power analysis. A 2.5-cm scapular spine autograft was harvested from all shoulders. A custom robot device was used to apply a 50-N compressive force to the glenohumeral joint. The humeral head was translated 10 mm posteroinferiorly at 30 degrees from the center of the glenoid at a rate of 1.0 mm/s in 6 consecutive conditions: (1) intact specimen, (2) intact with PABB, (3) posterior capsulolabral tear, (4) addition of the PABB, (5) removal of the PABB and repair of the capsulolabral tear (LR), and (6) addition of the PABB with LR. The maximum force required to obtain this translation was recorded. Paired t tests were performed to compare relevant testing conditions. RESULTS: Ten cadavers with a mean ± SD age of 54.4 ± 13.1 years and mean ± SD glenoid retroversion of 6.5 ± 1.0 degrees were studied. The PABB provided greater resistance force to humeral head translation compared to the instability state (instability, 29.3 ± 15.3 N vs PABB, 47.6 ± 21.0 N; P = .001; 95% confidence interval [CI], -27.6 to -10.0). When comparing PABB to LR, the PABB produced higher resistance force than LR alone (PABB, 47.6 ± 21.0 N; LR, 34.2 ± 20.5 N; P = .012; 95% CI, -23.4 to -4.1). An instability lesion treated with the PABB, with LR (P = .056; 95% CI, -0.30 to 20.4) or without LR (P = .351; 95% CI, -6.8 to 15.7), produced resistance forces similar to the intact specimen. CONCLUSIONS: A PABB is biomechanically effective at restoring the force required to translate the humeral head posteriorly in a cadaveric, posterior glenohumeral instability model. A posterior acromial bone block is a biomechanically feasible option to consider in patients with recurrent posterior instability. CLINICAL RELEVANCE: Augmentation of the posterior acromion may be a biomechanically feasible option to treat posterior shoulder instability.

An Approach to Robotic Testing of the Wrist Using Three-Dimensional Imaging and a Hybrid Testing Methodology.

Robotic technology is increasingly used for sophisticated in vitro testing designed to understand the subtleties of joint biomechanics. Typically, the joint coordinate systems in these studies are established via palpation and digitization of anatomic landmarks. We are interested in wrist mechanics in which overlying soft tissues and indistinct bony features can introduce considerable variation in landmark localization, leading to descriptions of kinematics and kinetics that may not appropriately align with the bony anatomy. In the wrist, testing is often performed using either load or displacement control with standard material testers. However, these control modes either do not consider all six degrees-of-freedom (DOF) or reflect the nonlinear mechanical properties of the wrist joint. The development of an appropriate protocol to investigate complexities of wrist mechanics would potentially advance our understanding of normal, pathological, and artificial wrist function. In this study, we report a novel methodology for using CT imaging to generate anatomically aligned coordinate systems and a new methodology for robotic testing of wrist. The methodology is demonstrated with the testing of 9 intact cadaver specimens in 24 unique directions of wrist motion to a resultant torque of 2.0 N·m. The mean orientation of the major principal axis of range of motion (ROM) envelope was oriented 12.1 ± 2.7 deg toward ulnar flexion, which was significantly different (p < 0.001) from the anatomical flexion/extension axis. The largest wrist ROM was 98 ± 9.3 deg in the direction of ulnar flexion, 15 deg ulnar from pure flexion, consistent with previous studies [1,2]. Interestingly, the radial and ulnar components of the resultant torque were the most dominant across all directions of wrist motion. The results of this study showed that we can efficiently register anatomical coordinate systems from CT imaging space to robotic test space adaptable to any cadaveric joint experiments and demonstrated a combined load-position strategy for robotic testing of wrist.

The 6-O'clock Anchor Increases Labral Repair Strength in a Biomechanical Shoulder Instability Model.

PURPOSE: To characterize the additive effect of a 6-o'clock anchor in the stabilization of a Bankart lesion. METHODS: Twelve cadaveric shoulders were tested on a 6-df robotic musculoskeletal simulator to measure the peak resistance force due to anterior displacement of 1 cm. The rotator cuff muscles were loaded dynamically. The test conditions consisted of the intact shoulder, Bankart lesion, Bankart repair (3-, 4-, and 5-o'clock anchors), and Bankart repair with the addition of a 6-o'clock anchor. A 13% anterior bone defect was then created, and all conditions were repeated. Repeated-measures analysis of variance was performed. RESULTS: In the group with no bone loss, the addition of a 6-o'clock anchor yielded the highest peak resistance force (52.8 N; standard deviation [SD], 4.5 N), and its peak force was significantly greater than that of the standard Bankart repair by 15.8% (7.2 N, P = .003). With subcritical glenoid bone loss, the repair with the addition of a 6-o'clock anchor (peak force, 52.6 N; SD, 6.1 N; P = .006) had a significantly higher peak resistance force than the group with bone loss with a Bankart lesion (35.2 N; SD, 5.8 N). Although the 6-o'clock anchor did increase the strength of the standard repair by 6.7%, this was not statistically significant (P = .9) in the bone loss model. CONCLUSIONS: The addition of a 6-o'clock suture anchor to a 3-anchor Bankart repair increases the peak resistance force to displacement in a biomechanical model, although this effect is lost with subcritical bone loss. CLINICAL RELEVANCE: This study provides surgeons with essential biomechanical data to aid in the selection of the repair configuration.

The passive biomechanics of the thumb carpometacarpal joint: An in vitro study.

The thumb carpometacarpal (CMC) joint facilitates multidirectional motion of the thumb and affords prehensile power and precision. Traditional methods of quantifying thumb CMC kinematics have been largely limited to range-of-motion (ROM) measurements in 4 orthogonal primary directions (flexion, extension, abduction, adduction) due to difficulties in capturing multidirectional thumb motion. However, important functional motions (e.g., opposition) consist of combinations of these primary directions, as well as coupled rotations (internal and external rotation) and translations. Our goal was to present a method of quantifying the multidirectional in vitro biomechanics of the thumb CMC joint in 6 degrees-of-freedom. A robotic musculoskeletal simulation system was used to manipulate CMC joints of 10 healthy specimens according to specimen-specific joint coordinate systems calculated from computed tomography bone models. To determine ROM and stiffness (K), the first metacarpal (MC1) was rotated with respect to the trapezium (TPM) to a terminal torque of 1 Nm in the four primary directions and in 20 combinations of these primary directions. ROM and K were also determined in internal and external rotation. We found multidirectional ROM was greatest and K least in directions oblique to the primary directions. We also found external rotation coupling with adduction-flexion and abduction-extension and internal rotation coupling with abduction-flexion and adduction-extension. Additionally, the translation of the proximal MC1 was predominantly radial during adduction and predominantly ulnar during abduction. The findings of this study aid in understanding thumb CMC joint mechanics and contextualize pathological changes for future treatment improvement.

Development of an implantable trapezium carpal bone replacement for measuring in vivo loads at the base of the thumb.

Understanding the loads that occur across musculoskeletal joints is critical to advancing our understanding of joint function and pathology, implant design and testing, as well as model verification. Substantial work in these areas has occurred in the hip and knee but has not yet been undertaken in smaller joints, such as those in the wrist. The thumb carpometacarpal (CMC) joint is a uniquely human articulation that is also a common site of osteoarthritis with unknown etiology. We present two potential designs for an instrumented trapezium implant and compare approaches to load calibration. Two instrumented trapezia designs were prototyped using strain gauge technology: Tube and Diaphragm. The Tube design is a well-established structure for sensing loads while the Diaphragm is novel. Each design was affixed to a 6-DOF load cell that was used as the reference. Loads were applied manually, and two calibration methods, supervised neural network (DEEP) and matrix algebra (MAT), were implemented. Bland-Altman 95% confidence interval for the limits of agreement (95% CI LOA) was used to assess accuracy. Overall, the DEEP calibration decreased 95% CI LOA compared with the MAT approach for both designs. The Diaphragm design outperformed the Tube design in measuring the primary load vector (joint compression). Importantly, the Diaphragm design permits the hermetic encapsulation of all electronics, which is not possible with the Tube design, given the small size of the trapezium. Substantial work remains before this device can be approved for implantation, but this work lays the foundation for further device development that will be required.

The multidirectional roles of the anterior oblique ligament and dorsoradial ligament of the thumb carpometacarpal joint.

The multidirectional biomechanics of the thumb carpometacarpal (CMC) joint underlie the remarkable power and precision of the thumb. Because of the unconfined nature of thumb CMC articulation, these biomechanics are largely dictated by ligaments, notably the anterior oblique ligament (AOL) and the dorsoradial ligament (DRL). However, the rotational and translational stabilizing roles of these ligaments remain unclear, as evidenced by the variety of interventions employed to treat altered pathological CMC biomechanics. The purpose of this study was to determine the effects of sectioning the AOL (n = 8) or DRL (n = 8) on thumb CMC joint biomechanics (rotational range-of-motion [ROM] and stiffness, translational ROM) in 26 rotational directions, including internal and external rotation, and in eight translational directions. Using a robotic musculoskeletal simulation system, the first metacarpal of each specimen (n = 16) was rotated and translated with respect to the trapezium to determine biomechanics before and after ligament sectioning. We observed the greatest increase in rotational ROM and decrease in rotational stiffness in flexion directions and internal rotation following DRL transection and in extension directions following AOL transection. The greatest increase in translational ROM was in dorsal and radial directions following DRL transection and in volar directions following AOL transection. These data suggest the AOL and DRL play complementary stabilizing roles, primarily restraining translations in the direction of and rotations away from the ligament insertion sites. These findings may inform future interventions or implant designs for pathological CMC joints.

The role of scapholunate interosseous, dorsal intercarpal, and radiolunate ligaments in wrist biomechanics.

Rupture to wrist ligaments predisposes the joint to degenerative changes. Scapholunate interosseous ligament (SLIL) rupture, especially when compounded by dorsal intercarpal ligament (DIC) and long radiolunate ligament (LRL) disruption, can cause carpal bone kinematic abnormalities. It is essential to delineate the role of these ligaments and their constraints on wrist range-of-motion (ROM) and center of rotation (COR). Wrist ROM and COR location were determined in 9 specimens using a six degree-of-freedom robotic musculoskeletal simulator in 24 directions of wrist motion for four experimental conditions: intact, and after sequential sectioning of the SLIL, DIC, and LRL. Sectioning the SLIL alone did not change wrist ROM in any direction (p > 0.10), while sectioning the SLIL and both the DIC and LRL caused significant increases in radial deviation, radial-extension, and ulnar-flexion ROM (p < 0.05). The COR of the intact wrist was located between the proximal third and middle third of the capitate, depending on the direction of wrist motion. While SLIL sectioning alone did not affect the COR, subsequent DIC sectioning led to a distal shift of COR in motions involving ulnar-extension relative to the intact condition. Additional sectioning of the LRL caused a proximal shift of COR in motions involving radial-flexion. A proximal shift implies a more dominant role of the radiocarpal joint, while a distal shift of the COR implies an increased role for the midcarpal joint. Understanding the role of ligaments on overall wrist mechanics is critical to devising new treatment strategies to restore wrist function.

Biomechanical Comparison of the Long Head of the Biceps Tendon Versus Conjoint Tendon Transfer in a Bone Loss Shoulder Instability Model.

BACKGROUND: Augmentation of Bankart repair with long head of the biceps tendon transfer has been previously described, although there is a paucity of literature describing its biomechanical effects. PURPOSE/HYPOTHESIS: The purpose of this study was to assess the effect of augmenting Bankart repair with either the conjoint tendon or the long head of the biceps tendon, both with and without subcritical (13%) glenoid bone loss. We hypothesized that, in a cadaveric model, augmenting Bankart repair with the long head of the biceps tendon would restore a greater degree of stability compared with augmenting Bankart repair with the conjoint tendon. STUDY DESIGN: Controlled laboratory study. METHODS: A total of 12 cadaveric shoulders were tested on a 6-degrees-of-freedom robotic musculoskeletal simulator to measure the peak resistance force due to an anterior displacement of 1 cm. The rotator cuff muscles were loaded during testing to simulate physiological conditions. The following test conditions were used for each specimen: (1) intact shoulder, (2) Bankart lesion with 13% anterior bone loss, (3) 13% bone loss with Bankart repair (anchors placed at the 3-, 4-, and 5-o'clock positions), (4) 13% bone loss with both Bankart repair and transfer of the long head of the biceps tendon, and (5) 13% bone loss with Bankart repair and transfer of the conjoint tendon. RESULTS: Labral repair with the addition of long head of the biceps tendon transfer had the greatest peak resistance force to anterior displacement among all groups (54.1 ± 5.5 N) and was significantly stronger than both standard Bankart repair by 16.3% (46.5 ± 7.6 N; P = .039) and the conjoint transfer procedure by 16.6% (46.4 ± 7.7 N; P = .008). CONCLUSION: Given the susceptibility of recurrent instability in shoulders with subcritical bone loss after isolated labral repair, it is important to consider augmenting Bankart repair in high-risk patients to avoid potential recurrence and the need for reoperations. Transferring the long head of the biceps tendon to the anterior glenoid represents one possible augmentation. CLINICAL RELEVANCE: We present biomechanical data for a relatively novel technique for augmenting capsulolabral repair strength in an anterior instability model with subcritical bone loss. These data represent biomechanical justification for the utilization of this relatively novel technique.

Critical Glenoid Bone Loss in Posterior Shoulder Instability.

BACKGROUND: There is currently no consensus regarding the amount of posterior glenoid bone loss that is considered critical. Critical bone loss is defined as the amount of bone loss that occurs in which an isolated labral repair will not sufficiently restore stability. PURPOSE: The purpose is to identify the critical size of the posterior defect. STUDY DESIGN: Controlled laboratory study. METHODS: Eleven cadaveric shoulders were tested. With the use of a custom robot device, a 50-N compressive force was applied to the glenohumeral joint, and the peak force that was required to translate the humeral head posteriorly and the lateral displacement that occurred with translation were measured. The defect size was measured as a percentage of the glenoid width. Testing was performed in 11 conditions: (1) intact glenoid and labrum, (2) simulated reverse Bankart lesion, (3) the reverse Bankart lesion repaired, (4) a 10% defect, (5) the reverse Bankart lesion repaired, (6) a 20% defect, (7) the reverse Bankart lesion repaired, (8) a 30% defect, (9) the reverse Bankart lesion repaired, (10) a 40% defect, and (11) the reverse Bankart repaired. RESULTS: Force and displacement decreased as the size of the osseous defect increased. The mean peak force that occurred with posterior displacement in specimens with a glenoid defect ≥20% and a reverse Bankart repair (13 ± 9 N) was significantly lower than the peak force that occurred in specimens with an isolated reverse Bankart repair (22 ± 10 N) ( P = .0451). In addition, the mean lateral displacement was significantly less in the specimens with a 20% glenoid defect and a reverse Bankart repair (0.61 ± 0.57 mm) compared with the lateral displacement that occurred in specimens with an isolated reverse Bankart repair (1.6 ± 0.78 mm) ( P = .0058). CONCLUSION: An osseous defect that is ≥20% of the posterior glenoid width remains unstable after isolated reverse Bankart repair. CLINICAL RELEVANCE: A bony restoration procedure of the glenoid may be necessary in shoulders with a posterior glenoid defect that is ≥20% of the glenoid width.

Comparison of a Distal Tibial Allograft and Scapular Spinal Autograft for Posterior Shoulder Instability With Glenoid Bone Loss.

BACKGROUND: Posterior glenoid bone deficiency can occur with recurrent glenohumeral instability. Glenoid reconstruction with a distal tibial allograft (DTA) has been reported to successfully restore contact pressures that occur during posterior glenohumeral translation. However, there are concerns regarding the risk of allograft resorption, availability, and costs. Extracapsular reconstruction using a scapular spinal autograft (SSA) has been reported as an alternative technique secondary to its anatomic location relative to the posterior shoulder and preferable autograft properties. There are no known prior biomechanical studies evaluating the scapular spine as an effective extracapsular graft choice. PURPOSE: To compare the efficacy of a DTA and SSA in restoring the stability of a glenoid with a large posterior bone defect compared with the intact native glenoid. STUDY DESIGN: Controlled laboratory study. METHODS: Ten cadaveric shoulders were tested. With the use of a custom KUKA robot, a 50-N compressive force was applied to the glenohumeral joint. The peak force required to translate the humeral head beyond the glenoid lip posteriorly as well as the lateral displacement that occurred during posterior translation were measured. Testing was performed in 5 conditions: (1) intact glenoid and labrum, (2) simulated reverse Bankart lesion, (3) 12-mm posterior glenoid defect, (4) glenoid defect reconstructed with a fresh DTA, and (5) glenoid defect reconstructed with an SSA. RESULTS: The mean glenoid width was 30 mm. The mean peak force and lateral displacement decreased significantly with a glenoid defect (0.99 ± 2.3 N and 0.06 ± 0.09 mm, respectively; P < .0001) compared with the intact glenoid (23.00 ± 9.7 N and 1.83 ± 0.70 mm, respectively; P = .0001). There was no significant difference between the peak force after reconstruction of the defect with a DTA (23.00 ± 7.4 N) and SSA (23.00 ± 7.7 N) when compared with the intact glenoid (P = .9999). There were no significant differences in the peak force between the 2 grafts (P = .9999). Additionally, both the DTA (1.04 ± 1.09 mm) and the SSA (1.02 ± 1.17 mm) demonstrated no differences in lateral displacement when compared with the intact glenoid (P = .2336 and .2043, respectively). There was no difference in lateral displacement that occurred between the DTA and SSA (P = .9999). CONCLUSION: Reconstruction of a large posterior glenoid defect with either a DTA or an SSA can effectively restore glenohumeral stability. CLINICAL RELEVANCE: This study supports the use of a DTA or SSA in posterior glenoid defect reconstruction. Clinical studies are needed to determine the long-term effects of utilizing such grafts.