BACKGROUND

Navigation tracker pins rigidly fixed to bone is a prerequisite for computer-assisted total knee arthroplasty. The first cases of fracture on navigation tracker pin sites have recently been reported.

HYPOTHESIS

The risk of fracture depends first on diaphyseal placement of the tracker pins, and second on "transcortical" tangential route of the tracker pin as well as failure to obtain rigid fixation.

MATERIAL AND METHODS

In a continuous series of 385 total knee arthroplasties, five patients (four women, one man) on average 73.2 years old (range: 65-79 years old) have sustained femoral fractures at the tracker pin site (incidence 1.3%). We investigated the demographic and radiological factors contributing to this complication.

RESULTS

The patients with fractures were obese or overweight with an average body mass index of 32.56 (range: 25.14-39.45) but this was not statistically different from the BMI of the population of patients without fractures. The average delay from arthroplasty to fracture was 12.6 weeks (range 7-21). The fracture was always preceded by several days of thigh pain and occurred after a minor trauma. The fractures were always simple originating from the tracker pin site. In four out of five cases, the tracker pins were placed in the diaphyseal femur, and in all cases at least one pin was transcortical. Closed endomedullary nailing or ORIF were performed in five cases, with no bone graft. Union was obtained with functional results that were equivalent to those before the fracture.

DISCUSSION

Fractures at the navigation tracker pin site are a complication which must be understood and explained to patients undergoing computer-assisted TKA because of the 1.3% incidence described in our series. The circumstances systematically associated with this type of fracture were: occurrence a certain amount of time after arthroplasty in obese patients who had pain before the fracture occurred. These fractures are favored by suboptimal placement of the tracker pins, especially in the lower diaphysis of the femur and transcortical fixation of at least one of the pins. Treatment included stable osteosynthesis and did not affect the results of total knee arthroplasty. The development of thigh pain some time after surgery in high risk patients (obesity, tracker pin site in the lower diaphysis, transcortical fixation) should suggest this diagnosis and weight-bearing should be avoided because these fractures are assimilated with stress fractures. Bicortical metaphyseal fixation should be the preferred tracker pin positioning for navigated total knee arthroplasty.

LEVEL OF EVIDENCE

Level IV retrospective study.