Lachman, Drawer and Pivot Shift in the Acute ACL Tear
A missed ACL tear leaves a young athlete with a knee that can give way on the next pivot, often with a meniscal tear already in the joint.
Key terminology
- Anterior cruciate ligament (ACL)
- Ligament inside the knee from the lateral femoral condyle to the front of the tibial plateau (stops the tibia sliding forward on the femur)
- Hemarthrosis
- Blood in a joint. After a twisting knee injury, swelling within hours (vs overnight for a simple effusion) points to a torn intra-articular structure
- Lachman
- Exam pulling the tibia forward with the knee slightly bent. Also the first test for a suspected ACL tear
- Endpoint
- The firm stop felt when an intact ligament tightens at the end of a test. A soft or absent endpoint suggests the ligament is torn
- Anterior drawer
- Exam pulling the tibia forward with the knee bent to 90° and the foot held down (reliable in chronic tears and unreliable in the swollen acute knee)
- Pivot shift
- Exam applying valgus and rotation while flexing the knee from extension (a clunk as the subluxed tibia reduces reproduces giving way)
- Bone bruise
- Marrow edema on MRI where bone struck bone during the injury. Its location maps the mechanism.
- Segond fracture
- Small avulsion off the lateral tibial plateau just below the joint line. It's rare, and when present an ACL tear is almost certain
Takeaways
- The anterior cruciate ligament (ACL) is the main restraint to the tibia sliding forward on the femur, and it usually tears without contact. It runs from the inner wall of the lateral femoral condyle to the front of the tibial plateau. The classic story is a plant, deceleration, pivot or landing with the knee near full extension, and 72% of 100 torn ACLs in athletes were non-contact (Boden et al, Orthopedics 2000;23:573-578, PMID 10875418).
- Rapid swelling after a twisting injury is blood, and blood in the knee means an ACL tear until proven otherwise. The ACL was disrupted in 72% of 85 acute traumatic hemarthroses, and a meniscal tear was found in 62%. In athletes the ACL was disrupted in 71 of 106 knees. A pop was reported just as often with an intact ACL (33% vs 36%), so the pop alone does not settle it (Noyes et al, J Bone Joint Surg Am 1980;62:687-695, PMID 7391091; Maffulli et al, J Bone Joint Surg Br 1993;75:945-949, PMID 8245089).
- The Lachman is the first test to do on an acute knee. The examiner pulls the tibia forward on a stabilized femur with the knee at 20-30° of flexion. A positive test shows more translation than the other side and a soft or absent endpoint. Pooled sensitivity was 85% across 28 studies, and awake sensitivity for acute tears was 81%, the highest of the 3 tests (Benjaminse et al, J Orthop Sports Phys Ther 2006;36:267-288, PMID 16715828; van Eck et al, Knee Surg Sports Traumatol Arthrosc 2013;21:1895-1903, PMID 23085822).
- The anterior drawer misses most acute ACL tears in the awake patient. It is done at 90° of flexion, which is hard to reach in a swollen, guarding knee. Awake sensitivity for acute tears was 38%, rising to 63% under anesthesia, and the test was positive in only 24% of torn ACLs examined without anesthesia (van Eck et al, Knee Surg Sports Traumatol Arthrosc 2013;21:1895-1903, PMID 23085822; Noyes et al, J Bone Joint Surg Am 1980;62:687-695, PMID 7391091).
- The pivot shift is the rule-in test, and it is rarely positive in an awake, guarding patient. It reproduces giving way, as the subluxed tibia reduces with a clunk under valgus and flexion. Pooled specificity was 98% with sensitivity of only 24%. Awake sensitivity for acute tears was 28%, rising to 73% under anesthesia, where specificity reached 98% (Benjaminse et al, J Orthop Sports Phys Ther 2006;36:267-288, PMID 16715828; van Eck et al, Knee Surg Sports Traumatol Arthrosc 2013;21:1895-1903, PMID 23085822).
- The classic Lachman numbers may be too optimistic. A 2022 meta-analysis excluding other ligament injuries found Lachman sensitivity of 81% and specificity of 85%, no better than the anterior drawer (83% and 85%), with sensitivity falling to 68% for complete tears. A 2015 review found 96% for complete tears awake, so the reviews disagree (Sokal et al, Knee Surg Sports Traumatol Arthrosc 2022;30:3287-3303, PMID 35150292; Leblanc et al, Knee Surg Sports Traumatol Arthrosc 2015;23:2805-2813, PMID 25763847).
- MRI confirms the tear and shows a signature bone bruise pattern. Pooled MRI sensitivity was 87% and specificity 93% for ACL tear across 14 studies. Bone bruises sit on the lateral femoral condyle and the posterolateral tibial plateau as the tibia shifts forward. In skiers with ACL tears, 81% had a lateral tibial plateau bruise and 40% a lateral femoral condyle bruise (Phelan et al, Knee Surg Sports Traumatol Arthrosc 2016;24:1525-1539, PMID 26614425; Speer et al, Am J Sports Med 1995;23:77-81, PMID 7726355).
- A Segond fracture on a plain film is a near-certain sign of ACL tear, and most ACL tears do not have one. It is a small avulsion off the lateral tibial plateau just below the joint line. It was present in only 9% of 151 ACL ruptures and was found in just 1 knee among a similar number of injuries with an intact ACL (Hess et al, Clin Orthop Relat Res 1994;(303):193-197, PMID 8194233).