Purpose: This comparative study of femoropatellar function in four types of total knee arthroplasty (TKA) was conducted to demonstrate the relation between the form of the femoropatellar articulation and the function outcome achieved with these prostheses.

Material and methods: Forty patients who had undergone first-intention TKA for primary degenerative joint disease were selected at random. Minimum follow-up was one year. The functional IKDC score was greater than 85/100. All TKA had been inserted without preservation of the posterior cruciate ligament. Four types of prostheses were used:

- posterior stabilised prosthesis with a fixed plateau, toric trochlea, cemented dome patella (n=10);

The following parameters were studied prospectively:

- pain assessed on a visual analogue scale;

Results: The kinematic data demonstrated a significant difference between:

- TKAs with a dome patella and those with an anatomic patella;

This difference basically involved the patellar tilt, lateral subluxation of the patella, and especially, the patellar trajectory between 20° and 90°, the toric trochlae with a dome patella having a more anterior trajectory than the normal knee.

The clinical and functional study showed that:

- the percentage of totally pain-free femoropatellar articulations was higher for the hollow anatomic trochlae (96%) than for the three other types (75%) (p = 0.04);

Discussion and conclusion: This study confirms the theoretic work reported by Walker and the clinical work reported by Andriacchi on the anterior curvature of the trochlae and the kinematic work reported by Stichl on the advantage of anatomic trochlae. These findings point out the advantage of the hollow anatomic trochlae where the patella is applied on the trochlear groove situated at the same depth as the normal trochlae, which is not the case with most TKAs. This advantage is seen by the absence of pain and by the propulsion when climbing stairs.

Purpose: The purpose of this work was to compare polyethylene (PE) wear between prostheses with similar function but different congruency of the femoral implant / PE insert and, as a corollary, the mobility of the plateau.

Material and methods: We studied two groups of knee prostheses: prostheses preserving both cruciate ligaments (n = 20), and stabilised prostheses without preservation of the cruciate ligaments (n = 20). Four representative samples of ten patients by type of total knee arthroplasty (TKA) were selected at random among a cohort of 105 patients operated on between 1994 and 1996 with a mean follow-up of five years. All patients were reviewed with AP and lateral radiographs, a view in the plateau plane, and goniometry. Using this random selection, patients in the two groups were comparable for operative age (69 years), diagnosis (degenerative disease), sex ratio, IKS score (> 80/100), and follow-up. The only difference between the two groups was the postoperative goniometry: 180±2° for fixed plateau; 178±3° for mobile plateau (p< 0.05).

Result: Penetration of the femur in the PE insert (U) (after correction for radiographic magnification) was: TKA two cruciates fixed plateau: U=3.5±1.5mm; TKA two cruciates mobile plateau: U=0±1mm (p< 0.001); TKA posterior stabilisation fixed plateau: U=2.5±1mm; TKA posterior stabilisation mobile plateau: U=0 mm (p< 0.001). A difference of more than 3° in the mechanical axis did not show detectable wear in the group of congruent prostheses while for fixed plateau prostheses, wear appeared when the mechanical axis was 180°.

Discussion: Few studies have compared PE wear of TKAs with identical form and function. The random selection allowed us to compare homogeneous groups of patients eliminating selection bias of the retrospective analysis and of the effect of patients lost to follow-up. The highly significant difference between the groups compared avoided potential ß risk. However the quality of the PE and its mode of sterilisation were not known with certainty for the tested implants.

Conclusion: This study demonstrates the importance of congruency as a factor reducing PE wear in TKA. This parameter is more favourable when the postoperative mechanical axis is perfectly corrected.