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1977 Publications

Lateral Instability of the Kneeby James A. Nicholas, MD*Director, Department of Orthopaedic Surgery and Founding Director Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital, New York, New York; Associate Professor, Clinical Orthopaedics at Cornell University Medical College and Attending Orthopaedic Surgeon at New York Hospital and the Hospital for Special Surgery. - last modified 2013-02-09 00:00
Orthopaedic Review Vol. VI, No. 5, May 1977


The tibial plateau is divided into a horizontal axis as well as a vertical axis and, thus, into four quadrants or comers. It is the movement of this plateau in relation to the femur that provides the reference axial points in the definition of knee instability.

The functional integrity of the posterior capsule is one of the most important of the structural dynamic components that control instability. It can be regarded as a half-bucket sleeve that runs from the medial to the lateral aspect of the tibia and femur, above and below the knee joint. It has specialized attachments, centrally, connecting it to the two menisci and the cruciates.

It also has numerous expansions on the posterior, medial and lateral sides, which convey the dynamic muscular supports to control extensive knee movement Kaplan described the lateral supporting structures as a ''quadruple complex." For convenience, the two menisci and cruciates, also form a ''central quadruple complex," and the medial capsular complex also has "four major components."

Knee instability may be congenital or acquired. Simple instability is instability of one plane. It is mild and can be lateral, medial or anteroposterior. The lateral capsular complex controls internal rotation and varus. The central complex, consisting of the menisci and two cruciate ligaments, controls the backward and forward motions of the tibia and femur, and also assists in the control of rotation, as well as the glide of the tibia on the femur in flexion and extension. The medial complex controls excessive motion in external rotation and valgus of the knee.

Together, these structures, along with the bony contours, and the patella quadriceps mechanism, allow the knee to have a hinge, rotary, rocking, rolling and gliding action. When any of the complexes become unstable or the muscles to them become weak, some part of all of the knee motion is affected, leading to degenerative joint disease.

The posterior capsule has special expansions, one of which is the arcuate ligament at the lateral side. It is the femoral-tibial anchor connecting fibrous attachments to the iliotibial band, the popliteal muscle, the biceps tendon, the gastrocnemius muscle and the lateral collateral ligament.