Cylindrical axis, not epicondyles, approximates perpendicular to knee axes.

BACKGROUND: The transepicondylar axis (TEA) is often used as a surrogate for the flexion-extension axis, ie, the axis around which the tibia moves in space, because of a belief that both axes lie perpendicular to the mechanical axis. However, studies suggest the cylindrical axis (CA), defined as a line equidistant from contact points on the medial and lateral condylar surfaces from 10(o) to 120(o) flexion, more closely approximates the axis around which the tibia moves in space. QUESTIONS/PURPOSES: We examined the TEA and CA angles relative to mechanical axes to determine whether one more consistently and closely approximates the surgical goal of orthogonality to the mechanical axis. METHODS: Three-dimensional (3-D) models were reconstructed from CT scans of five cadaver limbs. Three observers repeated three measurement sets to locate the TEA, CA, and femoral mechanical and tibial mechanical axes. Angles of the TEA and CA relative to the mechanical axes were calculated in two-dimensions (2-D) and as 3-D projections and compared for differences in magnitude and variance. RESULTS: Angles between CA and the mechanical axes were closer to 90° than the TEA in 2-D (92° versus 94° for the femur, 93° versus 94° for the tibia) and 3-D (88° versus 87° for the femur, 88° versus 86° for the tibia). Variance of the TEA was higher than the CA in 2-D. CONCLUSIONS: The CA forms angles more orthogonal to the mechanical axes of the thigh and leg than the TEA. CLINICAL RELEVANCE: Although we found a consistently greater deviation of the TEA from the mechanical axis than the CA with small differences, future studies will need to determine whether these differences are biomechanically or clinically important.

Effect of wrist posture on carpal tunnel pressure while typing.

Long weekly hours of keyboard use may lead to or aggravate carpal tunnel syndrome. The effects of typing on fluid pressure in the carpal tunnel, a possible mediator of carpal tunnel syndrome, are unknown. Twenty healthy subjects participated in a laboratory study to investigate the effects of typing at different wrist postures on carpal tunnel pressure of the right hand. Changes in wrist flexion/extension angle (p = 0.01) and radial/ulnar deviation angle (p = 0.03) independently altered carpal tunnel pressure; wrist deviations in extension or radial deviation were associated with an increase in pressure. The activity of typing independently elevated carpal tunnel pressure (p = 0.001) relative to the static hand held in the same posture. This information can guide the design and use of keyboards and workstations in order to minimize carpal tunnel pressure while typing. The findings may also be useful to clinicians and ergonomists in the management of patients with carpal tunnel syndrome who use a keyboard.

Guidelines for wrist posture based on carpal tunnel pressure thresholds.

OBJECTIVE: To develop work guidelines for wrist posture based on carpal tunnel pressure. BACKGROUND: Wrist posture is considered a risk factor for distal upper extremity musculoskeletal disorders, and sustained wrist deviation from neutral at work may be associated with carpal tunnel syndrome. However, the physiologic basis for wrist posture guidelines at work is limited. METHODS: The relationship of wrist posture to carpal tunnel pressure was examined in 37 healthy participants. The participants slowly moved their wrists in extension-flexion and radioulnar deviation while wrist posture and carpal tunnel pressure were recorded. The wrist postures associated with pressures of 25 and 30 mmHg were identified for each motion and used to determine the 25th percentile wrist angles (the angles that protect 75% of the study population from reaching a pressure of 25 or 30 mmHg). RESULTS: Using 30 mmHg, the 25th percentile angles were 32.7 degrees (95% confidence interval [CI] = 27.2-38.1 degrees) for wrist extension, 48.6 degrees (37.7 -59.4 degrees) for flexion, 21.8 degrees (14.7-29.0 degrees) for radial deviation, and 14.5 degrees (9.6-19.4 degrees) for ulnar deviation. For 25 mmHg, the 25th percentile angles were 26.6 degrees and 37.7 degrees for extension and flexion, with radial and ulnar deviation being 17.8 degrees and 12.1 degrees, respectively. CONCLUSION: Further research can incorporate the independent contributions of pinch force and finger posture into this model. APPLICATION: The method presented can provide wrist posture guidelines for the design of tools and hand-intensive tasks.

Variability of landmark identification in total knee arthroplasty.

Virtual reality is new technology that is finding application in many facets of orthopaedics. We will describe an application of virtual reality in orthopaedic research. Component placement in total knee arthroplasty depends on identification of anatomic landmarks about the knee. We surmised significant interobserver variability occurs in the identification of landmarks of the distal femur used in total knee arthroplasty. The results tested in virtual space show that certain anatomic landmarks used in total knee arthroplasty are not reliable. The significance of this observation is that landmark identification, an integral component of computer-assisted surgical navigation in total knee arthroplasty, represents a source of method error in an otherwise accurate and precise computer-assisted technique.

Thumb force and muscle loads are influenced by the design of a mechanical pipette and by pipetting tasks.

Work involving pipetting is associated with elevated rates of musculoskeletal disorders of the hand and wrist. The purpose of this study was to quantify thumb loading and muscle activity and determine if they varied among pipetting tasks. Fourteen experienced participants performed nine pipetting tasks while surface electromyography was measured for the extensor pollicis brevis, abductor pollicis longus, flexor pollicis longus, and abductor pollicis brevis muscles. For five tasks, participants used a pipette instrumented to measure the thumb force applied to the plunger. High-precision tasks significantly increased static muscle activity but reduced peak thumb force on average 5% as compared with low-precision tasks. Pipetting high-viscosity fluids increased peak thumb forces on average 11% as compared with pipetting low-viscosity fluids. Use of a latch pipette increased muscle activity of three muscles. We conclude that pipette design and pipetting tasks can influence applied thumb force and muscle activity. We recommend that pipettes be designed to limit applied peak forces and that pipette users be instructed in use patterns that will reduce applied forces. Actual or potential applications of this research include modifications to pipette designs and worker training in order to reduce hand pain associated with pipetting.