Introduction: The wear of orthopaedic biopolymers is recognised as a major factor in the failure of total joint replacements. Clinical wear data exists for acetabular cups manufactured from three biopolymers: ultra high molecular weight polyethylene (UHMWPE); poly tetra fluoro ethylene (PTFE); and polyacetal. The aim of this paper was to wear test these biopolymers and compare the results with clinical data.

Method and Materials: The biopolymers were tested using a modified, four-station, pin-on-plate wear rig [1]. In the tests, two of the four stations applied reciprocating motion and two applied multi-directional motion. Biopolymer pins articulated against stainless steel plates under a load of 40N. The lubricant consisted of 25% bovine serum and 75% distilled water. A standardised cleaning and weighing protocol was followed, and the biopolymer wear factors were calculated by dividing the volume lost by the product of the load and the sliding distance.

Discussion and Conclusions: Failed and retrieved UHMWPE acetabular cups have been reported as having a clinical wear factor of 2.1 x 10⁻⁶mm³/Nm [2]. However, UHMWPE cups which have been functioning well until removal at post-mortem have been said to show 45 to 69% less wear than revised UHMWPE cups [3]. Combining these values suggests clinical wear factors for functional UHMWPE in the range of 0.95 to 1.45 x 10⁻⁶mm³/Nm. This range fits well with the value of 1.1 x 10⁻⁶mm³/Nm shown in table 1 for UHMWPE under multi-directional motion. A clinical wear factor of 37 x10⁻⁶mm³/Nm has been calculated for PTFE acetabular cups [4]. When compared with the mean wear factor for PTFE pins under multi-directional motion obtained from the pin-on-plate rig, the match is remarkable. For polyacetal cups a mean volumetric wear of 136mm³/ year has been reported [5] and it has been calculated that explanted hip prostheses averaged 1.54 million cycles/year [2]. In polyacetal acetabular cups of 37mm diameter, an average sliding distance of 25mm/cycle can be calculated [6] and it has been said that an equivalent static load of 1000N applies [7]. Taking these four values permits a clinical wear factor for polyacetal cups of 3.5 x 10⁻⁶mm³/Nm to be calculated. This number compares well with the value of 3.8 x 10⁻⁶mm³/Nm seen for the polyacetal test pins under multi-directional motion. In summary, all three biopolymers subject to multi-directional motion exhibited clinically relevant values of wear.

Background and Literature Research: Fixed sagittal mal-rotation of pelvis is commonly encountered in patients with ankylosing spondylitis. The pelvis positioning for total hip arthroplasty in these patients can be a pitfall to an oblivious surgeon, and gives rise to mal-positioning of the acetabular component and subsequently leads to dislocation of the arthroplasty.

Objective: To quantify the effect of sagittal pelvic rotation on the positioning of acetabular component using three dimensional computer model.

Materials and Methods: Ten embalmed cadaveric pelvis with intact ligaments were scanned in 1 mm slices using computed tomogram (CT). The image reconstruction was done by the software “MIMICS” in microcomputer. The resulting three dimensional models can be rotated freely using “MIMICS.” Insertion of acetabular component was stimulated in different sagittal rotation of the pelvis. The ratio of the longitudinal to the transverse dimension of the obturator foramen was noted, and the uncovered area of the acetabular components was calculated.

Discussion: Pelvic rotation on the sagittal plane cannot be easily measured by radiographs. The shape of the obturator foramen on plane pelvic radiographs centered at pubic symphysis varies with the pelvic rotation on the sagittal plane and thereby serves as an indirect way to measure pelvic rotation. The shape of obturator foramen on plain radiographs therefore provided a guideline for patient positioning and the alignment of insertion of acetabular component during surgery.

Introduction: Normal axial alignment of the lower extremity is important to surgeons who perform reconstruction surgery of the knee. The data are, however, not available for Chinese adults.

Methods: The axial alignment of the lower extremity of twenty-five adult males and twenty-five adult females of southern Chinese origin was measured by using weight-bearing radiographs of the entire lower limb. The mean age of the male and female volunteers was twenty-four years and twenty-three years respectively. The axial alignment of the lower extremity was measured and the results were compared with two similar studies conducted in the United States.

Results: The medial inclination of the tibial plateau in the Chinese volunteers was greater than the commonly cited 3 degrees (with a mean of 5.4 ± 2.5 degrees for females and 4.9 ± 2.3 degrees for males). The extremities in Chinese volunteers were found to have a mean of 2.2 ± 2.5 degrees varus (females) and 2.2 ± 2.7 degrees varus (males).

Conclusions and Discussion: The medial inclination (knee joint obliquity) of the Chinese knee joint was significantly larger than Caucasian subjects. The higher knee joint obliquity exposes the cartilage of the Chinese knee to a higher shearing force and subsequently result in osteoarthritis. This may explain the racial difference in the ratio of knee to hip osteoarthritis. When performing total knee arthroplasty, a 5-degree, instead of the commonly cited 3-degree, external rotation of the femoral component may be required to obtain a rectangular flexion gap in total knee arthroplasty in Chinese patients.