Kristin D. Zhao

Dynamic contributions to superior shoulder stability.

It has been suggested that superior decentralization of the humeral head is a mechanical factor in the etiology of degenerative rotator cuff tears. This superior decentralization may be caused by muscular imbalance. The objective of this study was to investigate the contribution of individual shoulder muscles to superior stability of the glenohumeral joint. In 10 fresh frozen cadaver shoulders the tendons of the rotator cuff, teres major, latissimus, pectoralis major, deltoid and biceps were prepared. The shoulders were tested in a shoulder‐loading device in 0°, 30°, 60° and 90° of glenohumeral abduction. A constant superior force of 20 N was applied to the humerus. Tensile loads were applied sequentially to the tendons in proportion to their cross‐sectional areas and translations of the humeral head relative to the glenoid were recorded with a 3Space™ Fastrak system. Depression of the humeral head was most effectively achieved by the latissimus (5.6 ± 2.2 mm) and the teres major (5.1 ± 2.0 mm). Further studies should elucidate their possible in vivo role in the frontal plane force couple to counter balance the deltoid. The infraspinatus (4.6 ± 2.0 mm) and subscapularis (4.7 ± 1.9 mm) showed similar effects while the supraspinatus (2.0 ± 1.4 mm) was less effective in depression. Therefore, the infraspinatus and subscapularis should be surgically repaired whenever possible. The supraspinatus may be of less importance for superior stability than previously assumed.

The Effect of Seat Position on Wheelchair Propulsion Biomechanics

This study examined the effect of seat position on handrim biomechanics. Thirteen experienced users propelled a wheelchair over a smooth level floor at a self-selected speed. Kinetic and temporal-distance data were collected with the use of an instrumented rim and a motion analysis system. A custom-designed axle was used to change the seat position. We used repeated measures analysis of variance to evaluate if differences existed in the temporal-distance and kinetic data with change in seat position. Results showed that a shorter distance between the axle and shoulder (low seat height) improved the push time and push angle temporal variables (p < 0.0001). Tangential force output did not change with seat position. Axial and radial forces were highest in the lowest seat position (p < 0.001). Propulsion efficiency as measured by the fraction of effective force did not significantly change with seat position.

The Addition of Rotator Interval Closure After Arthroscopic Repair of Either Anterior or Posterior Shoulder Instability Effect on Glenohumeral Translation and Range of Motion

Background Although the use of rotator interval closure is frequently advocated as a useful supplement to shoulder instability repairs, the addition of a rotator interval closure after arthroscopic instability repair has not been fully investigated. Purpose The objective of this study was to investigate whether a rotator interval closure improves glenohumeral stability in an anterior and posterior instability shoulder model. Study Design Controlled laboratory study. Methods Fourteen fresh-frozen cadaveric shoulder specimens were dissected free of soft tissues, leaving the rotator cuff intact with simulated cuff loading. All specimens were mounted in a custom testing apparatus using infrared sensors to document glenohumeral translation and rotation. The specimens were then tested for stability in the following order: vented/subluxated state, after arthroscopic anterior (Group 1; 7 specimens) or posterior (Group 2; 7 specimens) instability repair with suture anchors, and then after rotator interval closure. For each of the 3 testing conditions, the following were measured: (1) external and internal rotation at neutral, (2) external and internal rotation at 90° of abduction, (3) posterior and anterior translation at neutral rotation (15 N and 25 N), (4) anterior translation at 90° of abduction and external rotation (Group 1; 15 N and 25 N), (5) posterior translation at 90° of flexion and internal rotation (Group 2; 15 N and 25 N), and (6) sulcus testing in neutral (7.5 N). Results Posterior stability was only improved after anchor capsulolabral repair (8.0 to 5.0 mm; P = .017, 25 N), but there was no improvement after rotator interval closure (5.0 to 4.6 mm; P = .453). However, anterior stability was improved after capsulolabral repair (8.6 to 4.0 mm; P = .016, 25 N) and also improved further by rotator interval closure (4.0 to 2.4 mm; P = .007). The mean loss of external rotation was significantly increased by the addition of the rotator interval closure in both neutral and abducted glenohumeral positions, with a mean external rotation loss of 28° in neutral (P = .013). The addition of a rotator interval closure did not improve sulcus stability (P = .4). Conclusion The addition of an arthroscopic rotator interval closure after posterior capsulolabral repair did not improve posterior stability; however, anterior stability was improved further after a rotator interval closure. Inferior stability was not improved. Arthroscopic rotator interval closure significantly decreased external rotation at both neutral and abducted arm positions. Clinical Relevance Arthroscopic closure may be beneficial in certain cases of anterior shoulder instability; however, posterior instability was not improved. Predictable losses of external rotation after rotator interval closure are of concern.

Shoulder impingement revisited: evolution of diagnostic understanding in orthopedic surgery and physical therapy

Abstract“Impingement syndrome” is a common diagnostic label for patients presenting with shoulder pain. Historically, it was believed to be due to compression of the rotator cuff tendons beneath the acromion. It has become evident that “impingement syndrome” is not likely an isolated condition that can be easily diagnosed with clinical tests or most successfully treated surgically. Rather, it is likely a complex of conditions involving a combination of intrinsic and extrinsic factors. A mechanical impingement phenomenon as an etiologic mechanism of rotator cuff disease may be distinct from the broad diagnostic label of “impingement syndrome”. Acknowledging the concepts of mechanical impingement and movement-related impairments may better suit the diagnostic and interventional continuum as they support the existence of potentially modifiable impairments within the conservative treatment paradigm. Therefore, it is advocated that the clinical diagnosis of “impingement syndrome” be eliminated as it is no more informative than the diagnosis of “anterior shoulder pain”. While both terms are ambiguous, the latter is less likely to presume an anatomical tissue pathology that may be difficult to isolate either with a clinical examination or with diagnostic imaging and may prevent potentially inappropriate surgical interventions. We further recommend investigation of mechanical impingement and movement patterns as potential mechanisms for the development of shoulder pain, but clearly distinguished from a clinical diagnostic label of “impingement syndrome”. For shoulder researchers, we recommend investigations of homogenous patient groups with accurately defined specific pathologies, or with subgrouping or classification based on specific movement deviations. Diagnostic labels based on the movement system may allow more effective subgrouping of patients to guide treatment strategies.

Dynamic inferior stabilizers of the shoulder joint

BACKGROUND The glenohumeral joint is soft-tissue balanced. However, few studies have focused on its dynamic inferior stabilizers. OBJECTIVE The objective of this study was to investigate the dynamic contributions of five shoulder muscles to inferior stability of the glenohumeral articulation in four joint positions. METHODS The anterior, lateral and posterior deltoid, supraspinatus, short head of biceps, coracobrachialis and long head of triceps from ten cadaveric shoulders were tested in 0 degrees, 30 degrees, 60 degrees and 90 degrees of glenohumeral abduction. A constant inferior force of 15 N was applied to the humerus. The tendons were loaded sequentially in proportion to their respective muscles cross-sectional area. Translations of the humeral head on the glenoid were recorded with a 3-Space tracking device. RESULTS The lateral deltoid (8.2 mm, SD 4.8 mm) was potentially most effective in superior translation of the humeral head followed by the posterior deltoid (7.7 mm, SD 4.8 mm). The coracobrachialis and short head of biceps had considerable capability to translate the humeral head superiorly (2.8 mm, SD 1.3 mm) while the supraspinatus showed the weakest effects (1.3 mm, SD 0.5 mm). RELEVANCE Strengthening exercises of the deltoid may be useful in the treatment of inferior glenohumeral instability, while the supraspinatus seems to be less important for inferior glenohumeral stability than previously assumed.

The Effect of Wrist Position on the Relative Motion of Tendon, Nerve, and Subsynovial Connective Tissue within the Carpal Tunnel in a Human Cadaver Model

The purpose of this study was to measure the effect of wrist position on the relative motion of the middle finger flexor digitorum superficialis (FDS) tendon, subsynovial connective tissue (SSCT), median nerve, and flexor retinaculum during simulated active finger motion. The relative motion of each tissue was measured by fluoroscopy in 10 human cadavers. Measurements were obtained for wrist positions of neutral (0 degree extension), 30 and 60 degrees of flexion, and 30 and 60 degrees of extension. The shear strain index (SSI) was defined as the difference in motion between two tissues (tendon, SSCT, or nerve) divided by tendon excursion, expressed as a percentage. The motion of the tendon, SSCT, and nerve in the 60 degree flexed position was significantly less than the motion in all other wrist positions (p < 0.001). The SSI at 60 degrees of flexion for tendon–SSCT and tendon–nerve were significantly increased compared with all other positions (p < 0.001). Because the SSCT and tendon are physically connected, a decrease in SSCT motion relative to the tendon would increase the shear strain on the SSCT with tendon motion. Thus, this result suggests that the SSCT may be predisposed to shear injury from activity done in 60 degrees of wrist flexion.

Dynamic CT technique for assessment of wrist joint instabilities

PURPOSE To develop a 4D [three-dimensional (3D) + time] CT technique to capture high spatial and temporal resolution images of wrist joint motion so that dynamic joint instabilities can be detected before the development of static joint instability and onset of osteoarthritis (OA). METHODS A cadaveric wrist was mounted onto a custom motion simulator and scanned with a dual source CT scanner during radial-ulnar deviation. A dynamic 4D CT technique was utilized to reconstruct images at 20 equidistant time points from one motion cycle. 3D images of carpal bones were generated using volume rendering techniques (VRT) at each of the 20 time points and then 4D movies were generated to depict the dynamic joint motion. The same cadaveric wrist was also scanned after cutting all portions of the scapholunate interosseus ligament to simulate scapholunate joint instability. Image quality were assessed on an ordinal scale (1-4, 4 being excellent) by three experienced orthopedic surgeons (specialized in hand surgery) by scoring 2D axial images. Dynamic instability was evaluated by the same surgeons by comparing the two 4D movies of joint motion. Finally, dose reduction was investigated using the cadaveric wrist by scanning at different dose levels to determine the lowest radiation dose that did not substantially alter diagnostic image quality. RESULTS The mean image quality scores for dynamic and static CT images were 3.7 and 4.0, respectively. The carpal bones, distal radius and ulna, and joint spaces were clearly delineated in the 3D VRT images, without motion blurring or banding artifacts, at all time points during the motion cycle. Appropriate viewing angles could be interactively selected to view any articulating structure using different 3D processing techniques. The motion of each carpal bone and the relative motion among the carpal bones were easily observed in the 4D movies. Joint instability was correctly and easily detected in the scan performed after the ligament was cut by observing the relative motion between the scaphoid and lunate bones. Diagnostic capability was not sacrificed with a volume CT dose index (CTDI(vol)) as low as 18 mGy for the whole scan, with estimated skin dose of approximately 33 mGy, which is much lower than the threshold for transient skin erythema (2000 mGy). CONCLUSIONS The proposed dynamic 4D CT imaging technique generated high spatial and high temporal resolution images without requiring periodic joint motion. Preliminary results from this cadaveric study demonstrate the feasibility of detecting joint instability using this technique.PURPOSE To develop a 4D [three-dimensional (3D) + time] CT technique to capture high spatial and temporal resolution images of wrist joint motion so that dynamic joint instabilities can be detected before the development of static joint instability and onset of osteoarthritis (OA). METHODS A cadaveric wrist was mounted onto a custom motion simulator and scanned with a dual source CT scanner during radial-ulnar deviation. A dynamic 4D CT technique was utilized to reconstruct images at 20 equidistant time points from one motion cycle. 3D images of carpal bones were generated using volume rendering techniques (VRT) at each of the 20 time points and then 4D movies were generated to depict the dynamic joint motion. The same cadaveric wrist was also scanned after cutting all portions of the scapholunate interosseus ligament to simulate scapholunate joint instability. Image quality were assessed on an ordinal scale (1-4, 4 being excellent) by three experienced orthopedic surgeons (specialized in hand surgery) by scoring 2D axial images. Dynamic instability was evaluated by the same surgeons by comparing the two 4D movies of joint motion. Finally, dose reduction was investigated using the cadaveric wrist by scanning at different dose levels to determine the lowest radiation dose that did not substantially alter diagnostic image quality. RESULTS The mean image quality scores for dynamic and static CT images were 3.7 and 4.0, respectively. The carpal bones, distal radius and ulna, and joint spaces were clearly delineated in the 3D VRT images, without motion blurring or banding artifacts, at all time points during the motion cycle. Appropriate viewing angles could be interactively selected to view any articulating structure using different 3D processing techniques. The motion of each carpal bone and the relative motion among the carpal bones were easily observed in the 4D movies. Joint instability was correctly and easily detected in the scan performed after the ligament was cut by observing the relative motion between the scaphoid and lunate bones. Diagnostic capability was not sacrificed with a volume CT dose index (CTDIvol ) as low as 18 mGy for the whole scan, with estimated skin dose of approximately 33 mGy, which is much lower than the threshold for transient skin erythema (2000 mGy). CONCLUSIONS The proposed dynamic 4D CT imaging technique generated high spatial and high temporal resolution images without requiring periodic joint motion. Preliminary results from this cadaveric study demonstrate the feasibility of detecting joint instability using this technique.

The contribution of the coronoid and radial head to the stability of the elbow

We undertook this study to determine the minimum amount of coronoid necessary to stabilise an otherwise intact elbow joint. Regan-Morrey types II and III, plus medial and lateral oblique coronoid fractures, collectively termed type IV fractures, were simulated in nine fresh cadavers. An electromagnetic tracking system defined the three-dimensional stability of the ulna relative to the humerus. The coronoid surface area accounts for 59% of the anterior articulation. Alteration in valgus, internal and external rotation occurred only with a type III coronoid fracture, accounting for 68% of the coronoid and 40% of the entire articular surface. A type II fracture removed 42% of the coronoid articulation and 25% of the entire articular surface but was associated with valgus and external rotational changes only when the radial head was removed, thereby removing 67% of the articular surface. We conclude that all type III fractures, as defined here, are unstable, even with intact ligaments and a radial head. However, a type II deficiency is stable unless the radial head is removed. Our study suggests that isolated medial-oblique or lateral-oblique fractures, and even a type II fracture with intact ligaments and a functional radial head, can be clinically stable, which is consistent with clinical observation.

The role of ankle ligaments and articular geometry in stabilizing the ankle.

BACKGROUND Ankle joint stability is a function of multiple factors, but it is unclear to what extent extrinsic factors such as ligaments and intrinsic elements such as geometry of the articular surfaces play a role. The purposes of this study were to determine the contribution of the ligaments and the articular geometry to ankle stability and to determine the effects of ankle position and simulated physiological loading upon ankle stability. METHODS Sixteen cadaveric lower extremities were studied in unloaded and with axial load equivalent to body weight. Anterior-posterior, medial-lateral translation and internal-external rotation tests were performed in neutral, dorsiflexion and plantarflexion ankle positions. Intact ankle stability was measured; ankle ligaments were serially sectioned and retested. FINDINGS For unloaded condition, the lateral ligament accounted for 70% to 80% of anterior stability and the deltoid ligaments for 50% to 80% of posterior stability. Both ligaments contributed 50% to 80% to rotational stability; however, the ligaments did not provide the primary restraints to medial-lateral stability. For loaded ankle condition, articular geometry contributed 100% to translational and 60% to rotational stability. The ankle was less stable in plantarflexion and more stable in dorsiflexion. INTERPRETATION The contribution of extrinsic and intrinsic elements to ankle stability is dependent upon the load and direction of force applied. This study underscores the importance of restoring soft tissues about the ankle to the anatomic condition during reconstruction operations for instability, trauma and arthritis.

The Biomechanical Effect of the Distal Interosseous Membrane on Distal Radioulnar Joint Stability: A Preliminary Anatomic Study

PURPOSE The distal interosseous membrane (DIOM) is a secondary stabilizer of the distal radioulnar joint (DRUJ) and has a considerably variable morphology. The purpose of this study was to investigate whether innate DRUJ stability is influenced by the anatomic variation of the DIOM. METHODS Ten fresh-frozen cadaver upper extremities were used in this study. The humerus and the ulna were affixed rigidly to a custom-made apparatus, with the elbow in 90° of flexion. Testing was performed by translating the radius in volar and dorsal directions relative to the ulna, with a 20-N applied force in neutral forearm alignment, 60° pronation, and 60° supination. Total translation of the radius was measured as DRUJ laxity. After the experiment, we investigated anatomic variation of the DIOM, especially regarding the existence of the distal oblique bundle (DOB), which is a notably thick fiber within the DIOM. We compared the DRUJ stability between the groups with and without the DOB. RESULTS The DOB was found in 4 of 10 specimens. The group with a DOB demonstrated a significantly greater DRUJ stability in the neutral position than the group without a DOB. In pronated and supinated forearm positions, no significant difference in DRUJ stability was obtained between the groups with and without a DOB. CONCLUSIONS Innate DRUJ stability in the neutral forearm position was greater in the group with a DOB than in those without a DOB. CLINICAL RELEVANCE This study suggests that considerable variation exists in DRUJ laxity and that it partially depends on anatomical variations of the DIOM.