Analysis of the morphology of the distal femur, and by extension of the femoral components in total knee arthroplasty (TKA), has largely been related to the aspect ratio, which represents the width of the femur. Little is known about variations in trapezoidicity (i.e. whether the femur is more rectangular or more trapezoidal). This study aimed to quantify additional morphological characteristics of the distal femur and identify anatomical features associated with higher risks of over- or under-sizing of components in TKA.

We analysed the shape of 114 arthritic knees at the time of primary TKA using the pre- operative CT scans. The aspect ratio and trapezoidicity ratio were quantified, and the post- operative prosthetic overhang was calculated. We compared the morphological characteristics with those of 12 TKA models.

There was significant variation in both the aspect ratio and trapezoidicity ratio between individuals. Femoral trapezoidicity was mostly due to an inward curve of the medial cortex. Overhang was correlated with the aspect ratio (with a greater chance of overhang in narrow femurs), trapezoidicity ratio (with a greater chance in trapezoidal femurs), and the tibio- femoral angle (with a greater chance in valgus knees).

This study shows that rectangular/trapezoidal variability of the distal femur cannot be ignored. Most of the femoral components which were tested appeared to be excessively rectangular when compared with the bony contours of the distal femur. These findings suggest that the design of TKA should be more concerned with matching the trapezoidal/ rectangular shape of the native femur.

Purpose of the study: Cementless stems completely coated with hydroxyapatite have confirmed long-term stability with 25 years follow-up for the oldest models (Furlong, Corail). Different bearings do not all have the same stability because of polyethylene wear debris. Unipolar replacements with a stable femoral implant raise the issue of changing the head on a used morse cone. Can a new alumina head be positioned on an old morse cone or must a metal head be used to avoid the risk of alumina fracture?

Material and methods: From 1993 to 2005, among the 228 revision hip prostheses in our centre, we reviewed retrospectively 79 unipolar replacements with implantation of a new ceramic head on a stable femoral implant (Corail). The other replacements were bipolar for 113 and unipolar with a metal head in 36. The reasons for the replacement were cup loosening (n=68), major polyethylene wear (n=11). Mean age was 71 years, 42 men for 37 women, mean time to reoperation 11 years (range 7–15). The stability of the femoral stem was examined on the intraoperative x-ray. The head was extracted with a mechanical extractor and the morse cone was protected throughout the operation. The quality of the titanium cone (12/14) was assessed visually as recommended by P. Hernigou (RCO 2003). Acceptable deterioration: intact cone or aspect of corrosion. Unacceptable deterioration: inclined cone, impacted cone, cone with wide truncation. A new alumina head was implanted on the cleaned, dry cone. The cup was replaced with a screwed socket (Tropic, Spirofit) and 64 polyethylene inserts for 15 alumina inserts.

Results: Sixty-one patients were reviewed. Seventeen patients died with no incident concerning the prosthesis; one patient was lost to follow-up. Mean follow-up since revision was nine years (range 2–15). Three cups were changed: two for recurrent dislocation and one for early migration, with a new head replacement. There were no head fractures and no cases of head decoaptation.

Conclusion: Although we have not had any incidents in this series, reimplantation of a new alumina head on an old morse code is not recommended by the manufacturers; but it is not prohibited if the cone is not macroscopically worn and remains clean and dry at reimplantation of the head. We now use alumina heads with an integrated 12/14 titanium sleeve. These sleeves need to be validated in the long term, but at the present time allow us to use large heads and an alumina-alumina bearing.

Purpose: Exposure to polyethylene debris and bearing wear is a common feature of total hip prosthesis inserted with or without cement. Osteolysis depends on the number of particles involved and their size. This continuous series of total hip arthroplasties using a Corail prosthesis with 12 years follow-up was studied to assess the course of osteolysis around the prosthesis and the resistance of hydroxyapatite coated implants to the aggression of wear debris.

Material and methods: One hundred fifteen patients underwent total hip arthroplasty in 1989 with a Corail prosthesis implanted by the same operator with a metal backed cup and a polyethylene insert. Mean age at implantation was 65 years. Review radiograms were available for 80 patients (38 women and 42 men). Twenty-seven patients had died, five answered a phone interview and three were lost to follow-up. The follow-up was 12 years and analysis was performed on the population with review data and x-rays.

Results: There was no case with a loosened stem and none of the patients underwent revision. Twelve cups were however revised for loosening with granuloma. Polyethylene insert wear was seen in 62% of the cases. There was a relationship between the amount of wear debris and the extent of the gramulomas on the femur. These granulomas were basically observed in zones I and VII. Bone defects were curetted and filled with grafts for the 12 cup revisions. The cup bearing was changed in young patients. Good bone healing was a constant finding on follow-up x-rays.

Discussion: Hydroxyapatite coated prostheses exhibit good long-term resistance to wear debris which causes osteolysis since none of the Corail femoral stems were destabilised. The granulomas remain localised in the proximal zone and do not descend along the stem or the cement as seen in cemented prostheses. The intimate contact between the recipient bone and the implant appears to form a barrier against migration of wear debris. It thus appears important to achieve good contact all along the stem and to use a totally hydroxyapatite coated implant.

Conclusion: Total hip arthroplasty without cement is spared the problem of polyethylene wear debris, but debris must be limited to prevent osteolysis. This observation questions the long-term appropriateness of metal-backed cups with polyethylene inserts and suggests that low friction arthroplasty or use of a hard bearing couple (Cr-Co-Cr-Co or alumina-alumina) should be preferred.