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Anterior Cruciate Ligament (ACL) Injury

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Overview

Exam Detail:

Key Revision Focus: KANON Trial outcomes (Osteoarthritis risk), Bundles (AM vs PL functions), Segond Fracture significance, Pivot Shift mechanism, and Return to Sport criteria (LSI >90%).

1. Clinical Overview

ACL Rupture is one of the most common and devastating knee injuries in sports medicine. The ACL is the primary restraint to anterior translation of the tibia and a secondary restraint to tibial rotation. Injury often leads to functional instability ("giving way") and is strongly associated with subsequent meniscal tears and early osteoarthritis.

Clinical Pearl:

The "Pop" and Swell: A non-contact twisting injury followed by an audible "pop" and immediate hemarthrosis (swelling within 1-2 hours) is an ACL tear until proven otherwise (>70% PPV). If swelling is delayed (next day), think Meniscus.

Key Concepts

  1. Anatomy: Two bundles.
    • Anteromedial (AM): Tight in Flexion. Controls AP translation.
    • Posterolateral (PL): Tight in Extension. Controls Rotation.
  2. Blood Supply: Middle Genicular Artery. Poor intrinsic healing potential (intrasynovial environment washes away clot).
  3. Mechanism: Valgus collapse + External Rotation of Tibia (Pivot Shift) + Flexion.
  4. Segond Fracture: Avulsion of the Anterolateral Ligament (ALL) from the proximal tibia. Pathognomonic for ACL tear.
  5. Unhappy Triad (O'Donoghue): ACL + MCL + Medial Meniscus (though Literal Triad is actually Lateral Meniscus more commonly).
  6. KANON Trial: No difference in patient-reported outcomes between early reconstruction vs rehab + optional delayed reconstruction.
  7. Graft Choice:
    • Bone-Patellar Tendon-Bone (BPTB): Gold standard for bone healing. Risk of anterior knee pain.
    • Hamstring (HT): Less donor site morbidity. Slower bone-tunnel healing.
    • Quadriceps: Rising popularity. Thick graft.
  8. Osteoarthritis: ACL Reconstruction does not significantly reduce the risk of long-term OA (the "die is cast" at injury). It stabilizes the knee to protect the menisci.

Clinical Pearls

  • Lachman Test: Most sensitive test for ACL (better than Anterior Drawer).
  • Pivot Shift: Best specific test for rotational instability. Hard to do in awake patient.
  • Female Athletes: 4-6x higher risk (Q-angle, Hormonal laxity, Neuromuscular firing patterns - "Landing in valgus").
  • Cyclops Lesion: Loss of extension post-op due to fibrous nodule in intercondylar notch.
  • RTS: Return to sport requires >9 months and passing functional tests. Biology of graft uptake takes time (ligamentization).

2. Epidemiology

  • Incidence: 200,000 cases/year in USA.
  • Age: Peak 15-25 years.
  • Gender:
    • Overall volume: Males (due to participation).
    • Risk per exposure: Females significantly higher (4-6x).
  • Sports: Soccer, Basketball, Skiing, Football (Cutting/Pivoting sports).

Risk Factors

  • Extrinsic:
    • Shoe-surface interaction (High friction).
    • Weather (Dry vs Wet).
  • Intrinsic:
    • Anatomic: Narrow intercondylar notch width index (Stenotic notch). Increased posterior tibial slope (>12 deg).
    • Hormonal: Pre-ovulatory phase (estrogen/relaxin).
    • Neuromuscular: "Ligament Dominance" (landing stiff/valgus) vs "Muscle Dominance".

3. Pathophysiology

Mechanism of Injury

  • Non-Contact (70%): Deceleration, cutting, or landing from a jump.
    • Knee in slight flexion (20°).
    • Valgus force.
    • Tibial Internal Rotation (or Femoral External Rotation).
    • Result: Subluxation of the lateral tibial plateau anteriorly (Pivot Shift).
  • Contact (30%): Direct blow to lateral knee (Valgus).

Associated Injuries

  • Bone Bruise ("Kissing Contusions"):
    • Found in >80% on MRI.
    • Location: Lateral Femoral Condyle (LFC) and Posterior Lateral Tibial Plateau (LTP).
    • From the pivot shift subluxation reduction.
  • Meniscal Tears:
    • Acute: Lateral Meniscus (trapped during subluxation).
    • Chronic: Medial Meniscus (restrains anterior translation in ACL absence -> fails over time).
  • MCL: Grade I/II common.
  • Chondral Injury: Impact damage.

Healing "Ligamentization"

ACL grafts (autografts) die after implantation and are repopulated by host cells.

  1. Necrosis: Graft is avascular (Day 0-3 weeks). Strongest mechanical point (fixation dependent).
  2. Revascularization: Synovial coverage (Week 6-12). Graft is WEAKEST here ("Mush").
  3. Cellular Proliferation: Fibroblasts invade.
  4. Remodeling: Collagen reorganization. Looks like normal ligament by 9-12 months.

4. Clinical Presentation

  • Pattern: "I was cutting left, felt a pop, knee gave way."
  • Immediate: Pain, inability to continue sport.
  • Swelling: Hemarthrosis develops rapidly (0-2 hours). (cf. Meniscus = 12-24 hrs).
  • Instability: Feeling of "trust issues" or buckling with turning.
  • Locking: Suggests bucket handle meniscus tear or stump impingement.

5. Clinical Examination

Compare to the normal side!

  1. Look:
    • Effusion (Ballottement).
    • Quadriceps atrophy (in chronic cases).
  2. Feel:
    • Joint line tenderness (Meniscus).
    • LCL/MCL tenderness.
  3. Move:
    • ROM check (Locked knee?). Loss of extension is common (Hamstring spasm or stump).
  4. Special Tests:
    • Lachman Test (Gold Standard Sensitivity):
      • Knee at 20-30° flexion. Stabilize femur, pull tibia anteriorly.
      • Positive: Increased translation (>3mm diff) and Soft Endpoint.
    • Anterior Drawer Test:
      • Knee at 90°. Sit on foot. Pull tibia.
      • Less Sensitive: Hamstrings can guard. Meniscus can wedge.
    • Pivot Shift Test (Gold Standard Specificity):
      • Replicates the giving way event.
      • Start in Extension. Apply Valgus + Internal Rotation. Flex the knee.
      • Clunk: At 20-40°, the subluxated tibia "reduces" back under the femur (IT Band pulls it back).
      • Grade III Pivot = High risk of failure.
    • Lever Sign (Lelli's Test):
      • Fist under calf. Push down on quad. Heel should rise. If heel stays down -> ACL rupture.

6. Investigations

X-ray

  • Often Acute Trauma Series (AP, Lateral, Skyline).
  • Segond Fracture: Small avulsion flake of Lateral Tibial Plateau (ALL insertion). 100% specific for ACL tear.
  • Arcuate Sign: Fibular head avulsion (PCL/PLC injury).
  • Deep Lateral Femoral Notch Sign: Impression fracture.

MRI (Gold Standard)

  • Sensitivity/Specificity >95%.
  • Direct Signs:
    • Discontinuity of fibers.
    • "Empty Notch" sign.
    • Horizontal orientation of ACL (tibial attachment avulsed).
  • Indirect Signs:
    • Bone Bruising: Middle of Lateral Femoral Condyle + Posterior Lateral Tibial Plateau.
    • Buckled PCL: PCL looks more curved because tibia is anterior.
    • Anterior Tibial Translation: >5mm relative to femur.

7. Management

Management is Patient Specific (Activity level, not just age).

ASCII Algorithm:

          ACL INJURY CONFIRMED
                   ↓
┌──────────────────────────────────────┐
│        INITIAL MANAGEMENT            │
│ - RICE / Crutches                    │
│ - "Pre-hab" (Regain ROM/Extension)   │
│ - MRI for associated pathology       │
└──────────────────────────────────────┘
                   ↓
┌──────────────────────────────────────┐
│           DECISION MAKING            │
├────────────────────┬─────────────────┤
│    CONSERVATIVE    │    SURGICAL     │
│ ("Copers")         │ (Reconstruct)   │
└─────────┬──────────┴────────┬────────┘
          ↓                   ↓
┌────────────────────┐ ┌────────────────────┐
│ INDICATIONS:       │ │ INDICATIONS:       │
│ - Low demand       │ │ - Instability (Pivot)
│ - No instability   │ │ - High demand sport│
│ - Advanced OA      │ │ - Repairable Menisc│
│ - Willing to mod   │ │ - Multi-ligament   │
│   activity         │ │                    │
└────────────────────┘ └────────────────────┘

1. Non-Operative Management

  • Candidates: Sedentary, low-demand, copers (no instability with ADLs), severe OA.
  • Protocol: Strengthening Quads/Hams, Proprioception.
  • Risk: Secondary meniscal tears if instability persists.

2. Surgical Reconstruction

  • Timing: Wait for Full Range of Motion and No Swelling (usu. 3-6 weeks). Operating on a stiff/swollen knee -> Arthrofibrosis.
  • Technique: Anatomical single bundle vs Double bundle (controversial). Drills tunnels in Femur and Tibia.
  • Graft Options:
    • BPTB (Patellar Tendon):
      • Pros: Bone-to-bone healing (fast). Gold standard for high-level athletes.
      • Cons: Anterior knee pain, patellar fracture risk.
    • Hamstring (Gracilis/Semitendinosus):
      • Pros: Strong (4-strand). Less kneeling pain.
      • Cons: Slower soft-tissue healing. Grade 3 Pivot may persist.
    • Quadriceps Tendon:
      • Pros: Thick graft, less harvest pain than BPTB.
      • Cons: Newer technique (learning curve).
    • Allograft (Cadaver):
      • Pros: No donor morbidity. Fast.
      • Cons: High failure rate in young active patients (>15% vs 4% auto). Slow biologic incorporation. Use only in >40s or revisions.

3. The Anterolateral Ligament (ALL)

  • In high-grade pivot shifts, adding an Lateral Extra-articular Tenodesis (LET) reduces failure rates (STABILITY Study).
  • Essentially acts as a "seatbelt" for rotation.

8. Complications

  • Graft Failure: 5-10%.
    • Causes: Technical error (tunnels too anterior), Trauma, Failure of graft incorporation.
  • Arthrofibrosis: Loss of extension. "Cyclops Lesion".
  • Infection: Septic arthritis (<1%).
  • Hardware irritation: Screws/buttons.
  • Saphenous Nerve Injury: Numbness over medial calf (Hamstring harvest).
  • Osteoarthritis: 50% develop OA at 10-20 years regardless of surgery. Most related to the initial bone bruise/chondral damage and meniscectomy.

9. Prognosis & Outcomes

  • Return to Sport (RTS):
    • Generally 65-80% return to pre-injury level.
    • Pro players: >85% return.
  • Re-rupture:
    • Graft rupture: 5%.
    • Contralateral ACL rupture: 5-10% (Highest risk period is first 2 years).
  • KANON Trial: At 2 and 5 years, no significant difference in KOOS scores between early reconstruction and rehab-first groups. Half of rehab group eventually opted for surgery.

10. Evidence & Guidelines

Guidelines

  • AAOS Guidelines: Moderate evidence for reconstruction in young/active patients to prevent instability.
  • NICE: MRI is the investigation of choice.

Landmark Trials

  • KANON Trial (Frobell et al, NEJM 2010): Randomized 121 active adults to Early ACLR vs Rehab (+/- delayed ACLR). No diff in outcomes at 2 or 5 years. Key Takeaway: You can safely try rehab first without compromising the knee. [PMID: 20660401]
  • MOON Cohort: Massive prospective multicenter study. Identified predictors of failure (Age, activity, allograft).
  • STABILITY Study (Getgood et al, 2020): ACLR + LET (Tenodesis) reduced graft failure by roughly 60% in high-risk young patients compared to ACLR alone. [PMID: 31905931]
  • Sanders et al: Meniscal preservation is the single biggest factor protecting against OA.

11. Patient Explanation

What is the ACL?

Think of the ACL as the central seatbelt of your knee. It stops the shin bone (tibia) from sliding forward out from under the thigh bone (femur) and controls twisting.

Do I need surgery?

Not everyone does. If you just want to walk, cycle, and swim ("straight line" activities), you might manage without it. However, if you want to play football, netball, or ski ("cutting" sports), the knee will likely buckle. If it buckles, you risk tearing the shock absorbers (meniscus).

Comparing Grafts

  • Hamstring: Using spare tendons from inner thigh. Strong, standard.
  • Patellar Tendon: Strip of tendon from kneecap with bone blocks. Very strong, heals fast, but kneeling can be painful. Good for pros.
  • Donor (Allograft): From a cadaver. Easier recovery, but higher chance of re-tearing in young people.

The Recovery Timeline

It takes 9-12 months to return to sport. The graft is actually weakest at 3 months (when it is remodeling). Feeling "good" at 4 months is a trap – the biology isn't ready.

12. References

  1. Frobell RB, Roos EM, Roos HP, Ranstam J, Lohmander LS. A randomized trial of treatment for acute anterior cruciate ligament tears. N Engl J Med. 2010 Jul 22;363(4):331-42. [PMID: 20660401] (KANON Trial).
  2. Getgood AMJ, et al. Lateral Extra-articular Tenodesis Reduces Failure of Hamstring Tendon Autograft Anterior Cruciate Ligament Reconstruction. Am J Sports Med. 2020 Feb;48(2):285-297. [PMID: 31905931] (STABILITY Trial).
  3. Claes S, Vereecke E, Maes M, Victor J, Verdonk P, Bellemans J. Anatomy of the anterolateral ligament of the knee. J Anat. 2013 Oct;223(4):321-8. [PMID: 23906341]
  4. Magnussen RA, et al. Graft size and patient age are predictors of early revision after anterior cruciate ligament reconstruction with hamstring autograft. Arthroscopy. 2012 Apr;28(4):526-34. [PMID: 22261140]
  5. Barenius B, et al. Increased risk of osteoarthritis after anterior cruciate ligament reconstruction: a 14-year follow-up study of a randomized controlled trial. Am J Sports Med. 2014 May;42(5):1049-57. [PMID: 24519183]
  6. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med. 1990 May-Jun;18(3):292-9. [PMID: 2372077]
  7. Grindem H, et al. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016 Jul;50(13):804-8. [PMID: 27162233]
  8. Paterno MV, et al. Biomechanical limb asymmetries at 12 months after anterior cruciate ligament reconstruction are associated with future injury risk. Am J Sports Med. 2016.
  9. Webster KE, Feller JA. Exploring the High Reinjury Rate in Younger Patients Undergoing Anterior Cruciate Ligament Reconstruction. Am J Sports Med. 2016 Nov;44(11):2827-2832. [PMID: 27159312]
  10. Ajuied A, et al. Anterior cruciate ligament injury and radiologic progression of knee osteoarthritis: a systematic review and meta-analysis. Am J Sports Med. 2014 Sep;42(9):2242-52. [PMID: 24275859]
  11. Segond P. Recherches cliniques et expérimentales sur les épanchements sanguins du genou par entorse. Prog Med (Paris). 1879. (Classic Description).
  12. Musahl V, Hoshino Y, Ahlden T, et al. The pivot shift: a global user guide. Knee Surg Sports Traumatol Arthrosc. 2012 Apr;20(4):724-31. [PMID: 22065099]
  13. Fredriksson M, et al. The "biology of graft maturation". Knee Surg Sports Traumatol Arthrosc. 2018.
  14. Wright RW, et al. Osteoarthritis Classification Scales: Interobserver Reliability and Arthroscopic Correlation. J Bone Joint Surg Am. 2014.
  15. Benjaminse A, et al. Neuromuscular training for the prevention of anterior cruciate ligament injuries. Sports Med. 2015.

13. Examination Focus

Common Exam Questions (FRCS/Boards)

  1. What is the blood supply to the ACL? (Answer: Middle Genicular Artery).
  2. Describe the Pivot Shift mechanism. (Answer: Valgus + Internal Rotation + Flexion causing anterior subluxation of lateral tibial plateau).
  3. What specific fracture is pathognomonic for ACL tear? (Answer: Segond Fracture / Avulsion of ALL).
  4. Summarize the 5-year findings of the KANON trial. (Answer: No diff in PROMs between early ACLR and Rehab+Delayed ACLR. OA rates similar).
  5. Why do we wait to operate? (Answer: To resolve inflammation and regain ROM, preventing Arthrofibrosis/Cyclops lesion).

Viva "Buzzwords"

  • "Pivot Shift"
  • "Hemarthrosis"
  • "Segond Fracture"
  • "Ligamentization"
  • "Cyclops Lesion"
  • "Copers vs Non-Copers"
  • "KANON Trial"

Common Pitfalls

  • Operating on a swollen/stiff knee: Guarantees stiffness. "Pre-hab" is mandatory.
  • Missing associated injuries: Especially PCL (PLC) or Meniscus root tears.
  • Using Allograft in young patients: High failure rate (don't do it in <25s).
  • Releasing to sport too early: <9 months equals high re-rupture risk. Time + Criteria needed.

At a Glance

EvidenceStandard
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Clinical Pearls

  • **Key Revision Focus:** KANON Trial outcomes (Osteoarthritis risk), Bundles (AM vs PL functions), Segond Fracture significance, Pivot Shift mechanism, and Return to Sport criteria (LSI &gt;90%).

Guidelines

  • NICE Guidelines
  • BTS Guidelines
  • RCUK Guidelines