SPT-007 · October 20, 2026 · Sports Medicine · Foundational

The MPFL in the First Patellar Dislocation

A first patellar dislocation tears the ligament that holds the kneecap in its groove, and a missed diagnosis or loose cartilage fragment can leave a teenager with a knee that keeps giving way.

Key terminology

Medial patellofemoral ligament (MPFL)
This band runs from the medial edge of the patella to the medial femur, inside the medial retinaculum. It is the main check against lateral patellar shift.
Adductor tubercle
This bump sits on the medial femur just above the medial epicondyle, where the adductor magnus tendon inserts. It is the landmark surgeons use to find the femoral end of the MPFL.
Apprehension test
The examiner pushes the patella laterally with the knee slightly bent. A positive test is the patient guarding or tensing the quadriceps to stop it.
Bone bruise
This MRI finding is marrow edema from bone striking bone, with no fracture line. Its location shows which bones collided.
Trochlea
This groove on the front of the distal femur is where the patella glides. Its lateral wall is the bony block to lateral patellar dislocation.
Trochlear dysplasia
The trochlea is shallow or flat instead of a deep groove. The patella then has little bone holding it in place.

Takeaways

  1. The medial patellofemoral ligament (MPFL) is the main soft-tissue restraint that stops the patella sliding laterally. It supplied 60% of the total restraining force at 20° of flexion in one cadaver study and 53% in another. It runs from the medial edge of the patella to the femur, attaching just distal to the adductor tubercle (Desio et al, Am J Sports Med 1998;26:59-65, PMID 9474403; Conlan et al, J Bone Joint Surg Am 1993;75:682-693, PMID 8501083; LaPrade et al, J Bone Joint Surg Am 2007;89:2000-2010, PMID 17768198).
  2. A teen with a bloody effusion after a twist may have a patella that dislocated and reduced on its own. Lateral patellar dislocation is the most common traumatic knee injury with hemarthrosis in children. It was not suspected before MRI in 73% of 26 transient cases. Look for medial parapatellar tenderness and a positive apprehension test (Askenberger et al, Am J Sports Med 2018;46:2328-2340, PMID 29847145; Kirsch et al, AJR Am J Roentgenol 1993;161:109-113, PMID 8517287; Lewallen et al, Am J Sports Med 2013;41:575-581, PMID 23339838).
  3. A bone bruise on both the medial patella and the lateral femoral condyle marks a patella that dislocated and relocated. The medial edge of the patella strikes the lateral femoral condyle as it snaps back into place. MRI showed lateral femoral condyle contusion in 80% and medial patellar contusion in 61% of 82 dislocations. An ACL tear bruises the lateral femoral condyle and the tibial plateau instead (Elias et al, Radiology 2002;225:736-743, PMID 12461254; Kirsch et al, AJR Am J Roentgenol 1993;161:109-113, PMID 8517287).
  4. The MPFL tears in nearly every first-time lateral patellar dislocation, and the tear site varies. MRI showed medial retinacular disruption or sprain in 96% of 26 transient dislocations. One series found the MPFL torn off the femur in 94% at surgery, and an MRI series of 195 knees found the patellar end torn most often (47%) (Kirsch et al, AJR Am J Roentgenol 1993;161:109-113, PMID 8517287; Sallay et al, Am J Sports Med 1996;24:52-60, PMID 8638754; Guerrero et al, Sports Med Arthrosc Rehabil Ther Technol 2009;1:17, PMID 19643022).
  5. A first dislocation is braced and rehabbed unless a loose osteochondral fragment needs fixing or removal. Osteochondral injury of the inferomedial patella showed on 70% of 82 MRIs, and loose bodies on 15%. Acute osteochondral injury was the reason for surgery in 2.3% of 609 first dislocations in a 21-year county cohort. Every first dislocation gets films to look for the fragment (Elias et al, Radiology 2002;225:736-743, PMID 12461254; Sanders et al, Sports Health 2018;10:146-151, PMID 28795924).
  6. Trochlear dysplasia and open growth plates are the strongest predictors of a second dislocation. Of 198 patients ≤18 y/o treated without surgery, 38% had recurrent instability, rising to 69% with both open physes and trochlear dysplasia. Recurrence was 17% after a first dislocation vs 49% with prior instability. Patella alta predicted recurrence in one series and not another (Lewallen et al, Am J Sports Med 2013;41:575-581, PMID 23339838; Jaquith and Parikh, J Pediatr Orthop 2017;37:484-490, PMID 26491910; Fithian et al, Am J Sports Med 2004;32:1114-1121, PMID 15262631).
  7. The patella is least stable near full extension, before it seats in the trochlea. In cadaver tests the patella subluxed most easily at 20° of flexion. The MPFL did the most work in the extended knee, since the medial retinaculum is tightest in full extension and slackens with flexion (Amis et al, Knee 2003;10:215-220, PMID 12893142).
  8. Early MPFL surgery may cut redislocation, and its effect on function is uncertain. A Cochrane review of 10 trials found 157 fewer redislocations per 1000 after surgery with no clear gain in knee scores, all at very low certainty. A pediatric trial with 14-year follow-up found recurrence in 71% after nonoperative care vs 67% after repair. Most first-time dislocations are still treated without surgery (Smith et al, Cochrane Database Syst Rev 2023;1:CD008106, PMID 36692346; Palmu et al, J Bone Joint Surg Am 2008;90:463-470, PMID 18310694).

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