The Slocum test assesses anteromedial rotary instability and anterolateral rotary instability of the knee. This means looseness both in a frontwards direction and in twisting movements..
How to perform the Slocum test
The patient lies on the unaffected side, with the unaffected knee slightly bent to prevent rolling. The pelvis of the upper affected leg is slightly rotated towards the examiner and the foot rests on the examination couch, while the knee is bent 10 degrees, allowing the hamstrings to relax.
In this position if there is a torn ACL the weight of the tibia will cause it to internally rotate and sublux, as there is no restraint.
The examiner stands at the side, with one hand above and one hand below the knee, the thumb of the lower hand at the back of the fibula while the fingers are over the joint line. The upper hand holds the thigh with the thumb over the lateral femoral condyle. The knee examiner gradually increases the knee flexion, and at about 40 degrees of flexion a palpable jerk will be felt as the subluxed tibia reduces back into normal position. This is considered a positive test
Peer-reviewed paperQuote from peer-reviewed paper:
"When anterointernal tibial luxation is present, a reduction phenomenon is felt as the knee passes into the 25 to 40 degree flexion range. This may occur as a sudden palpable and occasionally audible repositioning which is responsible for such terminology as a "pivot shift" or "jerk sign." "
Citation: Slocum DB, James SL, Larson RL, Singer KM. Clinical test for anterolateral rotary instability of the knee. Clin Orthop Relat Res. 1976 Jul-Aug;(118):63-9. PMID: 954292.
Interpreting a positive Slocum test
An isolated ACL tear will reveal itself on the anterior drawer test, while the Slocum test in addition identifies damage to the medial or lateral structures that also help to stabilise the knee.
These structures may include the superficial and deep medial collateral ligament and the posterior oblique ligament.
What this means in practice is that the surgeon needs to be aware to stabilise these structures during ACL reconstruction, or the reconstruction carries a risk of failure.
Peer-reviewed paperQuote from peer-reviewed paper:
"....following a combined ACL + MCL injury, [anteromedial rotatory instability] may persist if there is inadequate healing of both the [superficial and deep medial collateral ligament], and MCL deficiency increases the risk of ACL graft failure...." "
Citation: Ball S, Stephen JM, El-Daou H, Williams A, Amis AA. The medial ligaments and the ACL restrain anteromedial laxity of the knee. Knee Surg Sports Traumatol Arthrosc. 2020 Dec;28(12):3700-3708. doi: 10.1007/s00167-020-06084-4. Epub 2020 Jun 5. PMID: 32504158; PMCID: PMC7669770.