Valgus Laxity at 30° vs Full Extension
A knee that opens to valgus stress in full extension has more than a torn MCL, and missing the capsular or cruciate injury can leave the patient with a chronically unstable knee.
Key terminology
- Valgus
- This describes the lower leg angled outward from the knee, the knock-knee direction. A valgus force pushes the knee inward and stretches its medial side.
- Superficial MCL (sMCL)
- This band runs down the inner knee from the femur near the medial epicondyle to the upper tibia. It is the primary restraint to valgus opening of the knee.
- Posteromedial capsule
- This part of the joint capsule forms the back inner corner of the knee and is reinforced by the posterior oblique ligament. It is tight in full extension and slack at 30° of flexion.
- Valgus stress test
- This exam pushes the knee inward while the ankle is held out, opening the medial joint. It is done at 30° of flexion and at full extension and compared with the other knee.
- Grade III
- This grade means the ligament is torn completely. The knee opens to valgus stress at 30° and often at full extension as well.
Takeaways
- The superficial MCL is the main restraint to valgus stress at every angle of knee flexion. It runs from just behind the medial epicondyle of the femur to the tibia about 6 cm below the joint line. The deep MCL lies beneath it and attaches to the medial meniscus. The posterior oblique ligament reinforces the posteromedial capsule and backs up the sMCL from 0° to 30° (Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815; LaPrade et al, J Bone Joint Surg Am 2007;89:2000-2010, PMID 17768198).
- Valgus stress at 30° of flexion tests the superficial MCL on its own. At 30° the posteromedial capsule goes slack, so medial opening at that angle points to the sMCL. The examiner steadies the thigh, pushes the knee inward and compares the gap with the uninjured knee. Ecchymosis and painful swelling over the medial joint line are the usual findings (Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815).
- A knee that opens to valgus stress in full extension has more than an isolated MCL tear. Opening in extension means the sMCL and the posteromedial capsule are likely both torn. In cadavers a cut sMCL alone added only 1.7 mm of gapping at 0° and 3.2 mm at 20°. Cutting all the medial structures raised this to 6.5 mm and 9.8 mm, and cutting both cruciates as well reached 21.6 mm and 27.6 mm (LaPrade et al, Am J Sports Med 2010;38:330-338, PMID 19966093; Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815).
- Every grade III MCL tear needs a full cruciate exam because most come with another ligament injury. Fetto and Marshall found an 80% incidence of concomitant ligament injury with grade III tears. A later review put it near 78%, with the ACL involved in 95% of those. Associated injury, especially of the ACL, was linked to a worse prognosis (Fetto and Marshall, Clin Orthop Relat Res 1978;(132):206-218, PMID 679543; Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815).
- An isolated MCL tear, even grade III, is treated without surgery in a hinged brace with early motion. In 35 athletes with grade III tears, the mean knee score was 45.9 of 50 at 5.3 years, comparable with surgery. Fetto and Marshall had advised surgery for these tears in 1978, and later series reversed that. Surgery is kept for persistent instability, bony avulsion or a trapped ligament end (Reider et al, Am J Sports Med 1994;22:470-477, PMID 7943511; Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815; Fetto and Marshall, Clin Orthop Relat Res 1978;(132):206-218, PMID 679543).
- The MCL is the most common knee ligament injury, and an isolated tear comes from a valgus force on a flexed knee. The force can come from a contact blow to the outside of the knee or from a non-contact plant. Rotational mechanisms more often tear several ligaments at once. One 10-year sports clinic series recorded more ACL than MCL lesions (20.3% vs 7.9%), so the ranking depends on the population studied (Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815; Majewski et al, Knee 2006;13:184-188, PMID 16603363).
- MCL grade rises with the exam angle that opens. In the Fetto and Marshall system grade I has no valgus laxity, grade II opens at 30° and grade III opens at both 0° and 30°. On MRI grade I is an intact ligament with surrounding edema, grade II is a partial tear and grade III is a complete tear. AP and lateral films are taken in every patient to look for a bony avulsion (Encinas-Ullán et al, EFORT Open Rev 2018;3:398-407, PMID 30233815).
- When the ACL and MCL tear together, the ACL is reconstructed and the MCL usually heals without repair. A randomized trial of 47 patients with ACL plus grade III MCL tears found no difference in stability, function or return to activity whether the MCL was repaired or braced. A meta-analysis of 997 cases found the same. MCL surgery is added when valgus or anteromedial rotatory laxity persists after rehab (Halinen et al, Am J Sports Med 2006;34:1134-1140, PMID 16452264; Shultz et al, Am J Sports Med 2024;52:522-534, PMID 36960920).