Session IX - Tibia

Tibial Portal Placement: The Radiographic Correlate of the Anatomic Safe Zone

Paul Tornetta III, MD; John Tilzey, MD; Timothy McConnell, MD; David Casey, MD, Boston Medical Center, Boston, MA

Purpose: Several reports at recent OTA meetings have expounded the advantages of a lateral starting portal for tibial nailing. Anatomically, the safe zone for tibial nail portal placement on the superior surface of the tibia was reported be on the lateral plateau between the medial meniscus and the lateral articular surface. Likewise, nailing via a superolateral portal was demonstrated to diminish strain during nail insertion. The purpose of our study is to describe the location of the perfect starting portal for tibial nailing on the standard AP and lateral radiographic views used intraoperatively.

Methods: 20 cadaveric knees were used for the study. For each knee the experiment was performed in the same manner. The extensor mechanism was transected proximal to the patella and reflected distally on the patellar tendon. A K-wire was placed in the center of the anatomic safe zone equidistant from the medial meniscus and the lateral meniscus on the anterosuperior surface of the tibia. Standard AP and lateral radiographs were taken using a C-arm, and the images were printed. The following distances were measured on the AP radiograph: tibial plateau width, distance between the tibial spines, distance from the lateral edge of the plateau to the pin, and distance from the lateral spine (posterior spine) to the pin. The location of the starting portal (pin) was then calculated as a percentage of the tibial width and of the distance between the spines.

Results: On the AP radiograph, the center point of the anatomically centered tibial portal averaged 13% of the way from the lateral spine to the medial spine. In 25% of the knees, the portal was lateral to the lateral spine, and in 75% of the knees it was between the medial and lateral spines. It was never medial to the medial spine. Using the plateau width as a reference, the portal is located 44% ± 5% of the way across the plateau going >from lateral to medial. Stated differently, the ideal portal is centered 6% towards the lateral side of the plateau and is just medial to the lateral tibial spine on the AP x-ray. On the lateral radiograph, the starting point is located at the anterior margin of the articular surfaces.

Discussion: Recent emphasis has been placed on the anatomic and biomechanical characteristics of tibial nail portal location. This includes two studies presented at the 1998 OTA meeting. The anatomic location of the ideal portal with relation to the menisci was described. However, nailing is routinely performed without visualizing the anatomy of the tibial plateau. We sought to define the radiographic parameters of the ideal portal location with respect to the intraarticular structures of the knee when using a superior surface portal. Based on this study, the center point of the portal is located slightly medial to the lateral spine as visualized on the AP radiograph and at the anterior articular margin as visualized on the lateral radiograph. It is, as previously described, located lateral to the midline of the tibia by an average of 6% of the tibial plateau width. In five of the twenty specimens, the portal was located lateral to the lateral spine, but was never medial to the medial spine.

Conclusion: The ideal tibial portal based on previous anatomic and biomechanical studies is located, on average, just to the medial side of the lateral tibial spine on the AP view and at the anterior margin of the articular surface on the lateral view. These radiographic landmarks can be used when performing tibial nailing.