Abstract
Mobile bearings in knee arthroplasty carry the theoretical advantage of lower wearing prostheses. However, dislocating mobile bearings can be a significant issue in mobile bearing knee replacement arthroplasty. Our aim is to report our design alterations to the insert to address bearing spinout.
A total of 598 RBK mobile bearing total knee arthroplasties were performed by the senior author over a 10–year period. The standard bearing was subjected to three design changes to address spinout and increase flexion range. The first alteration involved a deeper dish with a higher anterior lip. Subsequently, a reduced footprint insert (RFI) was created. The final modification was a shaved off posterior rim to allow for greater flexion (high flex).
An overall bearing dislocation rate of 1.0% (6 out of 595) was obtained. Of these 595 knees, 132 were of the initial insert design, 194 were deep-dished inserts, 71 inserts were RFI, and 198 were high flex. There were four (3%) dislocations with the initial insert design and two (1%) dislocations in the final implant version. In our series the dislocated bearings have in all but one required revision to higher constrained prostheses. The mechanism of dislocation is speculated to be instability in flexion, leading to posterior loading of the insert and spinning out of the bearing. Most of the bearing subluxations have been medial but one was observed intra operatively to be a lateral extrusion. With respect to the two dislocations in the final implant design, one dislocation was attributed to a technical error of under sizing the insert. At revision surgery he was also found to have a disrupted MCL, which was repaired. He has had no further issues after the insert was upsized
. The cause of spin out in the second patient was speculated to be obesity and a diminished pre- operative range of movement. She required a revision to a higher constrained prosthesis.
Insert spinout has a multifactorial aetiology. The occurrence of spinout can be minimised by a combination of good surgical technique, such as balanced flexion and extension gaps and design modifications to the insert as we have instituted.
