FEMORAL HEAD REPLACEMENT DURING THE REVISION PROCEDURE: HOW CAN THE QUALITY OF THE MORSE CONE BE ASSESSED INTRA-OPERATIVELY?

Abstract

Purpose: The morse cone was introduced for industrial use as early as 1880 and for orthopaedic surgery in the 1970–1980s. It allows replacement of the head while preserving the femoral piece. This study was conducted to detail under what conditions a cone which has been implanted for several years can accept a new ceramic or metal femoral head.

Material and methods: Heads and cones from 30 prostheses removed after a mean nine years (5–16) were analysed by visual inspection as can be performed by the surgeon during a revision procedure. New femoral heads were implanted on the old cones. Implantations were repeated, first using ideal conditions, then non-ideal conditions (biological debris placed on the cone). The force necessary to destabilise the head-cone interface was compared with the force necessary for impaction of the heads, taking the principle that with lesser disassembly force, the risk of micromobility and stress fracture is greater. The qualitative results of the visual inspection (head and cone) were then compared with the quantitative force data.

Results: The cone left an imprint on the femoral head which was easily visualised on the alumina heads (white colour). A normal imprint was seen as a regular circular ring. Other configurations were considered to be an abnormal imprint (five of the thirty cases in this study). There were two types of anomalous cones (titanium or chromium/cobalt): visible anomalies (generally corrosion), and anomalies not recognised by the naked eye but found under the microscope or with a strong magnifying glass. When the cone had a normal aspect at visual inspection (naked eye) and the head had a normal imprint, the force required for disassembly (320–1260 N) of a new head from the old cone was correlated linearly (r=0.921; p< 0.01) with the force used for assembly (345–2345 N), for both ceramic and metal heads. This disassembly force did not vary with successive trials when there was no biological cellular debris on the cone. This force decreased significantly (p< 0.05) if the cone was placed in contact with cellular, blood or fatty debris. If the cone was cleaned with a compress after being placed in contact with cellular debris, the disassembly force returned to normal. The force necessary to achieve disassembly of a new head (ceramic or metal) was significantly decreased (p< 0.01) compared with the force of impaction if the head implanted before had an abnormal imprint or if the cone presented anomalies visible to the naked eye. Irrespective of the impaction force, fixation of metal heads was always better than that of alumina heads. The force necessary to achieve disassembly of an alumina head from an old morse cone was greater for metal titanium heads (compared with chromium/cobalt).

Discussion: Visual inspection should be performed to identify these anomalies intra-operatively on femoral heads and morse cones before reimplanting a ceramic or metal head on a femoral piece left in place. Precise analysis reveals the potential risks, depending on the different types of heads, the nature of the metal, and the aspect of the morse cone surface.