Biomechanical effects of kneeling after total knee arthroplasty.

BACKGROUND: Kneeling following total knee arthroplasty can be a difficult task, impairing the activities of patients to varying degrees. Little is known about the biomechanical effects of kneeling following total knee replacement. The objective of this study was to quantify the effects of kneeling on patellofemoral joint contact areas and pressures, knee joint reaction force, and patellar kinematics. METHODS: Total knee arthroplasties were performed on eight fresh-frozen cadaveric knees, and they were tested with use of a custom knee jig, which permits the simulation of physiologic quadriceps loading as well as the application of an anterior force to simulate kneeling. The knees were tested at flexion angles of 90 degrees , 105 degrees , 120 degrees , and 135 degrees with no anterior force (mimicking a squatting position) and with an anterior force application simulating double-stance kneeling and single-stance kneeling. Patellofemoral joint contact areas and pressures were measured with pressure-sensitive film, and the knee joint reaction force was measured with use of a six-degree-of-freedom load cell. Patellar kinematics were assessed with use of digital photographs tracking fixed markers on the patella. RESULTS: Compared with the condition without kneeling, both single-stance and double-stance kneeling demonstrated significant increases in patellofemoral contact area (p < 0.05) and pressure at all flexion angles (p < 0.05), with the exception of double-stance kneeling at 135 degrees of flexion. The resultant knee joint -reaction force increased with kneeling at all flexion angles. The compressive component of this force increased with kneeling for most conditions, while the lateral component of this force decreased significantly (p < 0.05) with kneeling only at 90 degrees , and the anterior component of this force increased significantly at all knee flexion angles (p < 0.05). Overall, kneeling had minimal changes on patellar tilt, with significant changes in patellar tilt seen only with kneeling at 120 degrees (p = 0.02 for double stance, and p = 0.03 for single stance). CONCLUSIONS: The findings of this study suggest that kneeling at a higher flexion angle (135 degrees ) after total knee arthroplasty has a smaller effect on patellofemoral joint contact area and pressure than kneeling at lower flexion angles (

Biomechanical effects of supraspinatus repair on the glenohumeral joint.

We repaired full-thickness rotator cuff tears in human cadaveric shoulder specimens. The purpose of this study was to determine whether a repaired supraspinatus will result in a change in joint forces, contact pressures and area, and position of the humerus relative to the glenoid compared with the pathologic and simulated complete-tear specimens. Force couples exist in the coronal plane between the deltoid and the inferior portion of the rotator cuff and in the transverse plane between the anterior cuff (subscapularis) and posterior cuff (infraspinatus and teres minor). This has served as a model for shoulder and rotator cuff research. Our model differs from previous studies in that the pectoralis major and latissimus dorsi/teres major were included because they are important contributors to shoulder function. Muscle force simulation was performed through a clamp, cable, and pneumatic system for the tendons of the rotator cuff, pectoralis major, and teres major/latissimus dorsi. Each specimen was tested in its native state of a full-thickness supraspinatus tear in 10 degrees and 60 degrees of abduction with neutral humeral rotation. In each position there were 2 loading conditions: (1) all muscles loaded with 60 N and (2) deltoid loaded with 90 N and 60 N for all others (3:2 ratio between the deltoid and supraspinatus). Pathologic tear, repaired, and simulated complete-tear conditions were subjected to the same testing sequence. After repair, there was an increase in percent inferior force in the 10 degrees abduction, 60-N loaded condition ( P = .02). The increase in percent inferior force may represent greater concavity-compression and spacer effect, which are both important functions of the supraspinatus. Contact pressure decreased in both the pathologic and simulated complete-tear conditions at 10 degrees abduction with 90-N deltoid loading ( P = .01). A corresponding increase in area was observed at this position for the pathologic tear condition only ( P = .01). This could represent greater concavity-compression, indicating that strengthening may provide a biomechanical benefit.