The postoperative patella height: a comparison of additive and subtractive high tibial osteotomy in correcting the genu varum.

INTRODUCTION: Intraligamentary correctional operations like a high tibial osteotomy were performed in genua valga to prevent later medial gonarthrosis especially in younger patients. An unwanted effect of this method seems to be the inferiorization of the patella. This is feared because of the complications in case of subsequent alloarthroplasty. Besides the classical Coventry method as a subtractive osteotomy the hemicallotasis has been established as a sustainable additive procedure. This means a gradual open wedge correction using an external fixateur.
OBJECTIVE: The aim of this study was to determine the position of the patella pre- and postoperatively and in follow-ups with subtractive versus additive intraligamentary high tibial osteotomies on the basis of five radiological parameters. It was expected that an additive osteotomy leads to an inferiorized patella position whereas a subtractive osteotomy leads postoperative to a higher position of the tibia.
METHOD: Between 1990 and 2001, 54 patients (61 legs) had undergone an operation due to a genu varum either by the subtractive osteotomy (n = 30) according to Coventry's method or the additive gradually hemicallotasis (n = 31) with an external fixator.
RESULTS: In coherence with the Coventry's osteotomy a significant inferiorization of the postoperative patella position with all five radiological parameters was observed, the hemicallotasis showed no operation-related significant alteration of the patella height. Instancing the Insall-Salvati Index there were four (12.9%) preoperative and three (9.7%) postoperative patella baja positions detected. Along with the subtractive osteotomy there were 5 preoperative patellae baja (16.7%) and 11 postoperative patellae baja (36.7%) positions. Furthermore a significant interrelation was noticed between the extent of the correctional angle and the postoperative alteration of the patella.
CONCLUSION: The results are surprising, contrary was expected. First this can be explained by its gradual, additive correctional property in contrast to the spontaneous correction by the conventional method according to Coventry, second by the postoperative treatment, which allows an early mobilization and active remedial gymnastics, provided an impact resistant osteosynthesis by a fixateur externe is given. In the case of the additive hemicallotasis an intraligamentary osteotomy is recommended. Technically expensive step cuts in order to osteotomize below the tuberositas tibiae are not necessary. Due to the low quota of complications and the small operative expense the continuous distraction is preferential to ad hoc correction. A postoperative patella baja position has not to be afraid in hemicallotasis.