Resurfacing metal-on-metal hip arthroplasty is currently showing promising clinical results. However there are concerns related to such implants, including the elevated levels of metal ions typically seen in patients. Valuable data can be obtained from explanted prostheses but due to their recent introduction few retrieval studies on resurfacing hip prostheses have been published. Five ASR hip resurfacing prostheses were revised due to pain. From two patients, head and cup were available for independent explant analysis. In the other three cases only femoral components were available. All were removed from female patients and all were revised to ceramic-on-ceramic hip prostheses. Post-operative radiographic measurements of cup inclination and ante-version were obtained using the EBRA software. The surface roughness values of the articulating surfaces of the explants were measured using a non-contacting profilometer. A co-ordinate measuring machine was used to measure the diameter of the head and the cup and thus the diametral clearance. The same measurements were then taken from a new unused ASR prosthesis and compared. Using elastohydrodynamic theory the minimum effective film thickness of the implant was calculated. In turn this allowed the lubrication regime to be determined. The average roughness values of the head and the cup of one implant were found to be 0.135microns and 0.058microns respectively, with a diametral clearance of 110microns. These results indicated that, at the time of removal, the prosthesis would have operated in the boundary lubrication regime. Other explants showed evidence of localised contact between the head and the rim of the acetabular cup, and these showed articulating surfaces with typical roughness values of between 0.025microns and 0.050microns. The new ASR had head and cup surface roughness values of 0.010microns and 0.012microns respectively and a diametral clearance of 87microns, implying that a new implant would operate under fluid film lubrication. All cups five were implanted with inclination angles over 45 degrees and anteversion over 25 degrees. These results suggest that components with high inclination and anteversion angles display greater than expected wear and may operate in boundary rather than fluid film lubrication which may eventually lead to early failure.
A lumbar laminectomy is a commonly performed surgical procedure for the decompression of neural structures. The aim of this human cadaveric study is to establish the extent of pars interarticularis remaining at each lumbar level when a laminectomy is performed to the medial edge of the pedicle. Seven human cadavers with intact lumbar spines were obtained for this study. The lumbar spine was dissected from the body and segmental disarticulation of each level was performed. The isolated lumbar levels had laminectomies performed exposing the neural canal. The vertical alignment of the laminectomy was orientated in line with the medial aspect of the ipsilateral pedicle. The remaining lateral pars interarticularis was measured with a calliper. The procedure was performed bilaterally at each isolated lumbar segment. Five males and two female cadavers with an age range of sixty-eight years to ninety-five years at the time of death. Fourteen lumbar segments of each respective level were available for study except at L5, where only twelve was possible due the presence of a transitional vertebra in one of the specimens. Taken to the nearest mms, the average width of the remnant pars interarticularis at the L1 level was 4 mm, range 3–6 mm (SD 0.95); L2 6 mm, range 5–7 mm (SD 0.77); L3 8mm, range 4–9mm (SD 1.34); L4 11mm, range 9–14 mm (SD 1.31) and L5 16mm, range 13–17 mm (SD 1.15). One way analysis of variance for each of the groups were performed to establish that the difference recorded was greater than that expected by chance (p<
0.05). The results predictably established the gradual narrowing of the pars interarticularis as the levels ascend cranially from L5. The medial wall of the pedicle could be used as an indirect means to establish a satisfactory remnant of the pars interarticularis following a laminectomy in the lower lumbar spine, at the levels of L3 to L5. However in the upper two levels direct visualisation of the pars is recommended.