In metal-on-metal (MoM) hip arthroplasties and resurfacings, mechanically induced corrosion can lead to elevated serum metal ions, a local inflammatory response, and formation of pseudotumours, ultimately requiring revision. The size and diametral clearance of anatomical (ADM) and modular (MDM) dual-mobility polyethylene bearings match those of Birmingham hip MoM components. If the acetabular component is satisfactorily positioned, well integrated into the bone, and has no surface damage, this presents the opportunity for revision with exchange of the metal head for ADM/MDM polyethylene bearings without removal of the acetabular component. Between 2012 and 2020, across two centres, 94 patients underwent revision of Birmingham MoM hip arthroplasties or resurfacings. Mean age was 65.5 years (33 to 87). In 53 patients (56.4%), the acetabular component was retained and dual-mobility bearings were used (DM); in 41 (43.6%) the acetabulum was revised (AR). Patients underwent follow-up of minimum two-years (mean 4.6 (2.1 to 8.5) years).Aims
Methods
This study was designed to emphasise the need for strict rotational alignment whilst performing a posterior sloping tibial resection during primary TKR. The normal posterior inclination of the bony tibia in the sagittal plane is around 10 degrees. The effect of the menisci reduces this to around 3 degrees. This reduces shear stress during flexion when compressive stress increases. In TKR it has been shown that tibial components inserted with zero posterior slope (ie. perpendicular in the sagittal plane to long axis of tibia) have an increased incidence of anterior subsidence. This has been shown to be due to the relative weakness of the anterior tibial bone. It is also known that this weakness increases with further resection. Therefore, most knee arthroplasty systems involve a posterior sloping tibial resection to minimize anterior bone loss. This resection, normally in the order of 7 degrees, needs to be made strictly in the AP plane. If a rotational error is introduced, the result will be to remove more bone from one plateau than the other and effectively produce a valgus or varus deformity. If the long axis alignment of the jig is also inaccurate, this will compound the error. The authors, using a series of sawbones calculated the resultant varus/valgus angulation produced by different degrees of rotational malalignment using posterior sloped cutting blocks of 3 and 7 degrees. We plan to confirm these findings by using a 3 D CT reconstruction of the human proximal tibia and computer software to simulate the cuts. We have shown that a rotational error of 30 degrees with a 7 degree cutting block will produce an angulation of up to 3 degrees measured in the coronal plane. Whilst not large in itself, this potential error should be highlighted, as a contributive factor in tibial component malalignment.
