Purpose of the study: Navigation systems have proven efficacy for implantation of total knee arthroplasty (TKA). Navigations have been accused of being complex, requiring a long learning curve. We compared the results obtained with the same navigation system in centers with experienced operators and centers with new operators.

Material and methods: Thirteen European centers participated in this prospective consecutive study. Inclusion criteria was indication for a TKA using a gliding prostheis with preservation of the posterior cruciate ligament. Four experienced cents(group A) with a mean experience of four years, and nine new centers (group B) with no prior experience participated in the study. The study concerned 403 TKA (182 in group A and 221 in group B). The main indications were primarily lateralized osteoarthritis. The navigation system was an imageless system based on intaoperative kinematic anatomic and kinematic analysis. A mobile plateau prosthesis was inserted. The following items were compared between the two groups: overall operative time and its variation over time, postoperative HKA, orientation of the femoral and tibial components in the ap and lateral views, complications and revisions.

Results: No significant difference was observed between the two groups for the preoperative items so comparison between the groups was licit. Correction of the frontal mechanical axis was satisfactory in 90% of patients in group A and 88% in group B (p> 0.05). There was no difference between the groups in quality of implantation for each prosthetic element on the ap and lateral views. There was no difference for rate of complications or reoperations. Longer operative time in group B disappeared after 15 implantations.

Discussion: The results from centers using navigation systems for prosthetic implantations shows that the performance in centers starting use is the same as in experienced centers. The only difference is an operative time slightly longer for the first 15 cases.

INTRODUCTION

Nowadays, longevity of total knee arthroplasties is very acceptable. Survivorship analyses demonstrate a success in a range of 80% to more than 95% over a period of more than ten years (1–4). However, long-term results largely depend, amongst other factors, on restoration of physiological alignment of the lower limb (5–11). Jeffery et al. (12) reported a three percent loosening rate over eight years when knees were correctly aligned whereas insufficient alignment lead to prosthetic loosening in 24 percent. Rand and Coventry (13) found a 90 percent survivorship rate at ten years when the mechanical axis was aligned in a range from nought to four degrees of valgus. Valgus position of more than four degrees or varus alignment resulted in only 71 percent and 73 percent of survivorship respectively.

Recently, computer aided instrumentation systems (14,15) became available and preliminary results of small series were reported (16–17).

The purpose of this study was to assess the accuracy of computer integrated instrumentation for knee alignment.

MATERIAL AND METHOD

The OrthoPilot^® represents a computer controlled image supported alignment system. A 3-D Optotrak™ camera localizes infra-red diodes fixed to rigid bodies within the surgical field. Thereby a spatial coordinate reference system is provided. The localizer is linked to a UNIX work station which performs the operative protocol using a graphical interface and a foot pedal. The rigid bodies are fixed to the bones by bicortical screws. An intraoperative kinematic analysis and various additional landmarks lead to definition of the centres of hip, ankle and knee joint and sizing of endoprosthetic components. With the use of LED-equipped alignment instruments the femoral and tibial resection planes are determined.

The OrthoPilot^® navigation system is not dependant on CT data and no additional preoperative planning is therefore necessary.

A prospective comparative multicentre study in five institutions, four in Germany and one in France, was carried out. 821 patients with primary tricompartimental knee arthroplasty using the SEARCH LC knee (B|Braun AESCULAP) were included in the study. The OrthoPilot^® Navigation system was used in 555 cases and 266 knees were implanted with the use of conventional instrumentation. At the three months follow-up alignment was assessed using standardized one leg stance radiographs with regard to the mechanical axis and the femoral and tibial angels in the coronal plane. For the lateral femoral and tibial angels standard lateral x-rays were used. Prosthetic alignment was verified by an independent observer.

RESULTS

The radiographically assessed results were subdivided into three groups. An error of ± one degree in the radiographical measurements and small deviations caused by the play of surgical instruments have to be considered. With respect to the femoral and tibial angels in the ap and lateral view the group of very good clinical results was, therefore, defined in the range between ninty degrees and ± two degrees. Deviations of three and four degrees from the optimum were classified as being clinically acceptable. Aberrations of more than four degrees were classified as outliers. When measuring the mechanical axis deviations from fully precise femoral and tibial angels may add up. For this reason zero degrees ± three degrees were rated as a very good result, deviations of four to five degrees were considered to be acceptable and alignment beyond five degrees from the optimum was classified as an unsatisfactory result.

Mechanical axis:

35. 2% of the navigated cases were aligned at exactly zero degrees. This was achieved in only 24. 4% of the manual cases. 88. 6% of cases using navigation and 72. 2% in the manual group showed zero degrees and varus or valgus angles of up to three degrees. 8. 9% and 18. 1% of cases respectively showed deviations of four or five degrees of valgus or varus alignment representing an acceptable clinical result. In only 2. 5% of the navigation group aberrations of more than five degrees occurred. The rate of dissatisfying results was 9. 8% in the manual group.

Femoral axis (coronal plane):

In the navigation group 48. 1% of cases showed an alignment at exactly 90 degrees which was the case in only 33. 5% of the control group. Altogether, in 89. 4% of the navigated cases a very good result was observed. In the conventionally instrumented cases only 77. 1% very good results were found. There were 1. 6% outliers beyond the limits of four degrees in the navigation group in comparison to 4. 9% amongst the control cases.

Femoral axis (sagittal plane):

Very good results with up to two degrees of deviation from a ninety degree position were obtained in 75. 5% of navigated cases and 70. 7% of manual cases. 37. 3% and 34. 6% respectively showed an ideal alignment of exactly ninety degrees. Unsatisfactory results were observed in 9. 5% of the navigated cases and 9. 4% of the manual cases.

Tibial axis (coronal plane):

58. 7% of the computer assisted and 40. 6% of the reference cases were exactly aligned at rectangles. All in all, in 91. 9% navigated and only 83. 5% manual cases a very good result was obtained. Only 1. 1% outliers had to be observed in the navigation group whereas 3. 4% unsatisfactory results were registered with manual technique.

Tibial axis (sagittal plane):

44. 3% of the navigated cases and only 26. 7% of cases in the control group were aligned perpendicular to the dorsal tibial cortex, thus showing no posterior slope. Altogether, 81. 3% could be classified as very good clinical results in the computer assisted group. The corresponding rate of the manual group was 69. 9%. Equivalent values of 8. 6% in the navigation group and 8. 3% in the reference group were registered beyond the limits of four degrees deviation.

The additional operation time for the use of the navigation system is calculated between eight and ten minutes after having passed through the learning curve.

CONCLUSIONS

Knee navigation facilitates proper alignment of endoprosthetic components and with the use of the Ortho-Pilot^® system results are clearly more favourable in comparison to conventional instrumentation technique. In addition, the data obtained from literature demonstrate that the use of this navigation system contributes to reducing outliers in number. With the learning curve the OrthoPilot^® alignment system proved to gain in reliability.

Deviations from perfect alignment are still difficult to be classified into surgical or technical deficiencies.

Many technical and software improvements which were introduced in the meantime will, in addition, contribute to reliability and time saving.

Comparative studies with different navigation systems are not yet available. They might allow an even more profound insight into the possibilities and advantages or disadvantages of computer assisted knee alignment.

LITERATURE

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