Isokinetic strength differences between patients with primary reverse and total shoulder prostheses: muscle strength quantified with a dynamometer.

BACKGROUND: Range of motion after total shoulder arthroplasty is better than after reverse shoulder arthroplasty, however with similar clinical outcome. It is unclear if this difference can only be found in the different range of motion or also in the force generating capacity. QUESTIONS: (1) are isokinetically produced joint torques of reverse shoulder arthroplasty comparable to those of total shoulder arthroplasty? (2) Does this force-generating capacity correlate with functional outcome? METHODS: Eighteen reverse shoulder arthroplasty patients (71years (SD 9years)) (21 shoulders, follow-up of 21months (SD 10months)) were recruited, 12 total shoulder arthroplasty patients (69years (SD 9years)) (14 shoulders, follow-up of 35months (SD 11months)). Pre- and post-operative Constant-Murley scores were obtained; two isokinetic protocols (ab-/adduction and ex-/internal rotations) at 60°/s were performed. FINDINGS: Twelve of 18 reverse shoulder arthroplasty patients generated enough speed to perform the test (13 shoulders). Mean ab-/adduction torques are 16.3Nm (SD 5.6Nm) and 20.4Nm (SD 11.8Nm). All total shoulder arthroplasty patients generated enough speed (14 shoulders). Mean ab-/adduction torques are 32.1Nm (SD 13.3Nm) and 43.1Nm (SD 21.5Nm). Only 8 reverse shoulder arthroplasty patients (9 shoulders) could perform ex-/internal rotation tasks and all total shoulder arthroplasty patients. Mean ex-/internal rotation torques are 9.3Nm (SD 4.7Nm) and 9.2Nm (SD 2.1Nm) for reverse shoulder arthroplasty, and 17.9Nm (SD 7.7Nm) and 23.5Nm (SD 10.6Nm) for total shoulder arthroplasty. Significant correlations between sub-scores: activity, mobility and strength and external rotation torques for reverse shoulder arthroplasty. Moderate to strong correlation for sub-scores: strength in relation to abduction, adduction and internal rotation torques for total shoulder arthroplasty. INTERPRETATION: Shoulders with a total shoulder arthroplasty are stronger. This can be explained by the absence of rotator cuff muscles and (probably) medialized center of rotation in reverse shoulder arthroplasty. The strong correlation between external rotation torques and post-operative Constant-Murley sub-scores demonstrates that external rotation is essential for good clinical functioning in reverse shoulder arthroplasty.

The active and passive kinematic difference between primary reverse and total shoulder prostheses.

BACKGROUND: Reverse shoulder arthroplasty (RSA) and total shoulder arthroplasty (TSA) effectively decrease pain and improve clinical outcome. However, indications and biomechanical properties vary greatly. Our aim was to analyze both active and passive shoulder motion (thoracohumeral [TH], glenohumeral [GH], and scapulothoracic [ST]) and determine the kinematic differences between RSAs and TSAs. METHODS: During 3 range-of-motion (ROM) tasks (forward flexion, abduction, and axial rotation), the motion patterns of 16 RSA patients (19 shoulders), with a mean age of 69 ± 8 years (range, 58-84 years), and 17 TSA patients (20 shoulders), with a mean age of 72 ± 10 years (range, 53-87 years), were measured. The mean length of follow-up was 22 ± 10 months (range, 6-41 months) for RSA patients and 33 ± 18 months (range, 12-87 months) for TSA patients. Kinematic measurements were performed with a 3-dimensional electromagnetic tracking device. RESULTS: All patients showed better passive than active ROM. This difference was significantly larger for RSA patients than for TSA patients (TH in sagittal plane, 20° vs 8° [P = .001]; GH in sagittal plane, 16° vs 7° [P = .003]; TH in scapular plane, 15° vs 2° [P < .001]; GH in scapular plane, 12° vs 0° [P < .001]; and ST in scapular plane, 3° vs -2° [P = .032]). This finding also showed that in the scapular plane, TSA patients showed hardly any difference between active and passive ROM. Furthermore, TSA patients had 16° to 17° larger active TH motion, 15° larger active GH motion, and 8° larger active ST motion compared with RSA patients. The GH-ST ratios showed similar figures for both types of prostheses. CONCLUSION: TSA patients have larger active TH motion because in the scapular plane, they completely use the possible GH motion provided by the prosthetic design. This larger active ROM in TSA patients only applies for elevation and abduction, not for axial rotation or passive ROMs.

Shoulder kinematics is not influenced by external load during elevation in the scapular plane.

The current study aimed to compare the shoulder kinematics (3D scapular orientation, scapular angular displacement and scapulohumeral rhythm) of asymptomatic participants under unloaded and loaded conditions during unilateral shoulder elevation in the scapular plane. We used a repeated-measures design with a convenience sample. Eleven male participants with an age range of 21-28 years with no recent history of shoulder injury participated in the study. The participants performed isometric shoulder elevation from a neutral position to approximately 150 degrees of elevation in the scapular plane in intervals of approximately 30 degrees during unloaded and loaded conditions. Shoulder kinematic data were obtained with videogrammetry. During shoulder elevation, the scapula rotated upwardly and externally, and tilted posteriorly. The addition of an external load did not affect 3D scapular orientation, scapular angular displacement, or scapulohumeral rhythm throughout shoulder elevation (P > .05). In clinical practice, clinicians should expect to observe upward and external rotation and posterior tilt of the scapula during their assessments of shoulder elevation. Such behavior was not influenced by an external load normalized to 5% of body weight when performed in an asymptomatic population.

Comparison of shoulder resultant net moment between three different exercises and load conditions.

Different exercises are prescribed by physiotherapists and despite the popular use of elastic resistance, few studies have investigated the effect of such rehabilitation tools on shoulder resultant net moment (RNM). The aim of the present study was to compare shoulder RNM peak values and the respective angle of occurrence during three shoulder rehabilitation exercises: 1) elevation in the scapular plane; 2) flexion; and 3) abduction when performed in three different load situations: 1) without external load; 2) with dumbbells (DB); and 3) with elastic resistance. Twenty-one healthy subjects participated in the study. Kinematic data were obtained by means of an inverse dynamic model. A two-way ANOVA was used for data analysis (α < 0.05). The highest RNM peak for abductor and external rotator muscles was during shoulder abduction exercise and for flexor muscles was during flexion and elevation in the scapular plane. The DB load condition was associated with highest RNM peak values for all muscle groups. This study presents differences among three exercises and load situations for RNM peak values and angle of occurrence. Furthermore, it also presents theoretical rationale for load progress and selection of exercises for shoulder rehabilitation management. Clinicians should consider it, when prescribing strengthening exercises for shoulder rehabilitation.