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Comparison of Cutaneous and Transosseous Electromagnetic Position Sensors in the Assessment of Tibial Rotation in a Cadaveric Model

David P. Magit, MD^*, Michelle McGarry, MS, James E. Tibone, MD^* and Thay Q. Lee, PhD^,

From the ^* Kerlan Jobe Orthopaedic Clinic, Los Angeles, California, and the Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System, Long Beach, and University of California, Irvine, Long Beach, California

Address correspondence to Thay Q. Lee, PhD, Department of Orthopaedic Surgery, VA Long Beach Healthcare System, 5901 E. 7th St. (09/151), Long Beach, CA 90822 (e-mail: tqlee{at}med.va.gov).

Background: Recent studies have highlighted the growing interest in validating anatomic anterior cruciate ligament reconstructions. A simple method of measuring tibial rotation is necessary to provide an objective clinical assessment of restoration of normal knee kinematics after various anterior cruciate ligament reconstructive techniques.

Objective: To validate a new method of measuring tibial rotation by comparing cutaneous with transosseous electromagnetic position sensors during a simulated standard knee examination.

Study Design: Controlled laboratory study.

Methods: Eight thawed, fresh-frozen cadaveric knee specimens with skin and soft tissues preserved were mounted on the femoral side in neutral rotation by a rigid clamp, allowing 6 degrees of freedom of the knee joint. With the knee fixed at 30° of flexion, a series of maximal manual internal and external tibial rotations were performed and measured with an electromagnetic tracking system that measures 6 degrees of freedom in a Cartesian coordinate system. During each series of measurements, a cutaneous transmitter was fixed overlying the tibial tuberosity. Simultaneously, a second transducer was rigidly fixed to a trans-osseous pin placed just distal to the tibial tubercle. Measurements were repeated at 90° of flexion. Differences in measurements were assessed.

Results: No significant differences were found with maximal internal and external rotation between cutaneous and transosseous measurements at 30° of knee flexion (13.0° vs 14.5°, P = .4) or at 90° of flexion (11.2° vs 12.9°, P = .5). Correlation (R) between cutaneous and transosseous measurements at 30° was .97 (P = .00009) and at 90° was .99 (P < .00001). The accuracy of cutaneous measurements using transosseous as the known was 1.6° ± 1.3°. Repeatability of cutaneous measurements was 0.8° ± 0.4°. The repeatability of transosseous measurements was 1.0° ± 0.5°.

Conclusion: No significant differences were found at either 30° or 90° of flexion when measuring tibial rotation using cutaneous versus transosseous electromagnetic position sensors.

Clinical Relevance: The ability to measure knee rotation using cutaneous electromagnetic position sensors represents a promising new method for assessing various clinical conditions and surgical outcomes.

Key Words: tibial rotation • electromagnetic position sensors • cadaveric model