A clinical and radiographic approach for establishing proper tibial tubercle transfer when using quad active femoral nerve stimulation.

Background: The tibial tubercle-trochlear groove measurement (TT-TG), which measures the lateral offset of the tibial tubercle relative to the trochlear groove of the femur, has been utilized as an intraoperative tool to help establish maximum patellofemoral congruency in patients who suffer from patellar instability. We have previously published our approach of establishing how far to transfer the tibial tubercle using intraoperative femoral nerve stimulation in order to achieve congruency from 0-30° of flexion. The technique and clinical outcomes have previously been published in this journal and elsewhere. Here we describe the use of the TT-TG distance to determine how far to transfer the tibial tubercle to achieve our goals and have found that it varies according to the clinical exam features.

Purpose: We intended to determine the effectiveness of using the preoperatively established TT-TG to predict the degree of intraoperative me- dialization of the tibial tubercle to achieve our goal of establishing dynamic congruency of the patella in the trochlear groove when using the previously described femoral nerve stimulation method of estimating dynamic tracking of the patella.

Methods: From the study group of patients used in other publications, we examined 20 knees in 18 patients who had a history of recurrent lateral dislocations and underwent a Fulkerson tibial tubercle transfer. Each knee was dynamically assessed preoperatively by obtaining an MRI at 30° of flexion and complete hyperextension while voluntarily contracting their quadriceps. These were then compared to the intraoperative transfer of the tibial tubercle required to achieve maximum congruency when the femoral nerve was stimulated. We then looked at the preoperative TT-TG measurement to determine its role in predicting what was required at achieving congruency in the context of the quad active MRI findings.

Results: Thirteen knees preoperatively demonstrated a positive J-sign defined as the patella subluxated greater than or equal to 5 mm lateral in full extension compared to 30° of flexion. In these patients, the TT-TG was accurate if the distance medialized was 1:1 with the measured TT-TG. In 7 out of the 20 knees, the patella demonstrated a false negative J-sign where the patella was radio- graphically subluxated at 30° of flexion as well as at hyperextension. In this group, the TT-TG underestimated the transfer required for congruency on average 5mm even when using the 1:1 ratio.

Conclusions: The preoperative use of the J-sign is of value when determining the role of the TT-TG measurement and estimating the distance required to intraoperatively achieve congruency when using the femoral nerve stimulation technique. Those that demonstrated a positive J-sign of 5mm or greater, a 1:1 ratio of TT-TG to medialization is most reliable at establishing congruency of the patellofemoral joint. Whereas, those that demonstrated a false negative J-sign even the 1:1 ratio remains inadequate at producing congruency and more medialization is required.

Level Of Evidence: Level III, Retrospective Observational/Comparative Study.