41.1 Meniscal Tears, Ligamentous Knee Injuries & Patellofemoral Pain

Functional Anatomy & Knee Joint Biomechanics

The knee joint is a modified bicondylar hinge joint comprising two distinct functional articulations: the tibiofemoral joint (bearing substantial axial compressive loads) and the patellofemoral joint (acting as a frictionless pulley system for the extensor mechanism). Osseous congruity between the convex femoral condyles and the flat tibial plateaus is minimal; therefore, dynamic and static stability relies almost entirely on an intricate capsuloligamentous network and muscular force couples.

Static Ligamentous Stabilizers

1. Anterior Cruciate Ligament (ACL):
   • Anatomy: Originates from the posteromedial aspect of the lateral femoral condyle within the intercondylar notch and courses obliquely anteromedially to insert into the anterior intercondylar fossa of the tibial plateau, anterior to the intercondylar eminence.
   • Functional Bundles: Composed of two distinct functional bundles named for their tibial insertion:
     • Anteromedial (AM) Bundle: Taut in knee flexion; provides primary resistance against anterior tibial translation.
     • Posterolateral (PL) Bundle: Taut in knee extension; provides primary resistance against rotational torque and hyperextension.
   • Biomechanical Role: Provides 85% of total passive resistance against anterior tibial translation and acts as the secondary restraint against internal tibial rotation and excessive valgus/varus stress.
   • Mechanism of Injury: Non-contact deceleration, cutting, or pivoting maneuvers with the knee in slight flexion (10° to 20°), dynamic valgus collapse, and external tibial rotation (e.g., landing from a rebound in basketball, plant-and-cut in soccer). Less commonly, direct hyperextension or contact blows.
   • Clinical Hallmarks: The patient frequently experiences or hears an audible "pop" (present in >70% of acute ruptures), feels the knee "give way," and develops a rapid, tense hemarthrosis within 2 to 4 hours of injury. An acute post-traumatic hemarthrosis developing within 4 hours has an associated ACL rupture rate exceeding 70% to 80% (with patellar dislocation, osteochondral fractures, and peripheral meniscal tears accounting for the remainder).

                      CRUCIATE & COLLATERAL LIGAMENT MECHANICS

   Ligament   Primary Biomechanical Action         Classic Injury Mechanism       Gold Standard Exam
   ───────────────────────────────────────────────────────────────────────────────────────────────────
   ACL        Resists anterior tibial translation  Non-contact deceleration,      Lachman Test
              (85%) & internal rotation            pivoting, valgus collapse      (Sens: 85-95%)

   PCL        Resists posterior tibial translation Dashboard impact to tibia;     Posterior Drawer
              (95%)                                fall on flexed knee (plantar)  (Sens: 90%)

   MCL        Resists valgus opening stress &      Contact blow to lateral knee;  Valgus Stress (30°)
              external rotation                    rotational stress              (End-point laxity)

   LCL        Resists varus opening stress &       Direct varus force to medial   Varus Stress (30°)
              external rotation                    knee; hyperextension           (PLC evaluation)
   ───────────────────────────────────────────────────────────────────────────────────────────────────

1. Posterior Cruciate Ligament (PCL):

   • Anatomy: Originates from the anterolateral aspect of the medial femoral condyle and inserts into the posterior intercondylar fossa of the tibia, approximately 1 cm below the articular margin. It is significantly thicker, broader, and stronger than the ACL.
   • Biomechanical Role: Provides 95% of total passive restraint against posterior tibial displacement.
   • Mechanism of Injury: Classically, a "dashboard injury" sustained during a motor vehicle collision where the proximal anterior tibia impacts the dashboard with the knee flexed to 90°. In sports, it occurs from a direct fall onto a flexed knee with the foot in plantarflexion (striking the tibial tuberosity), or severe forced hyperextension.
   • Clinical Hallmarks: Often presents with milder joint effusion and less immediate pain than ACL tears. Patients complain of vague posterior knee aching and instability when descending inclines or stairs.

2. Medial Collateral Ligament (MCL):

   • Anatomy: Divided into two structural layers: the superficial MCL (primary static valgus stabilizer extending from the medial femoral epicondyle to the medial tibial metaphysis, 4 to 5 cm distal to the joint line) and the deep MCL (a thickening of the joint capsule firmly attached to the medial meniscus via meniscofemoral and meniscotibial coronaries).
   • Biomechanical Role: Resists valgus stress (maximal at 30° knee flexion) and external tibial rotation.
   • Mechanism of Injury: Direct contact blow to the lateral aspect of the knee producing a violent valgus stress (e.g., clipping in football), or non-contact external rotational torque on a planted foot.

3. Lateral Collateral Ligament (LCL) & Posterolateral Corner (PLC):

   • Anatomy: An extracapsular cord-like structure running from the lateral femoral epicondyle to the head of the fibula, distinct from the joint capsule and lateral meniscus.
   • Biomechanical Role: Primary restraint against varus opening stress and external rotation.
   • Associated Structures: The PLC encompasses the LCL, popliteus tendon, and popliteofibular ligament. The common peroneal (fibular) nerve winds around the fibular neck in intimate proximity to the LCL insertion; injury to this region carries a high risk of peroneal nerve neuropraxia or transection (manifesting as foot drop and sensory loss over the first dorsal web space).

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Diagnostic Physical Examination Maneuvers

A systematic physical examination of the injured knee requires acute hemarthrosis evacuation if tense, gentle patient positioning, and immediate comparison to the contralateral uninjured extremity.

Cruciate Ligament Examination

[!IMPORTANT] LACHMAN TEST VS. ANTERIOR DRAWER TEST: SENSITIVITY PITFALLS

• The Lachman test is the undisputed gold standard physical examination maneuver for acute ACL rupture, possessing a pooled sensitivity of 85% to 95% and specificity >95%.
• The Anterior Drawer test has a dramatically inferior sensitivity in the acute setting (50% to 60%). Hamstring muscle spasm (due to acute pain), acute tense hemarthrosis blocking flexion to 90°, and the mechanical wedging effect of the posterior horn of the medial meniscus against the tibial plateau all falsely mask anterior tibial translation during the Anterior Drawer maneuver.

                  DIAGNOSTIC CRUCIATE & COLLATERAL MANEUVERS

      Maneuver          Ligament Tested        Technique & Pathognomonic Finding
   ════════════════════════════════════════════════════════════════════════════════════════
   Lachman Test         ACL                    Knee flexed 20°-30°; examiner stabilizes
                        (Gold Standard)        distal femur with one hand, translates tibia
                                               anteriorly with other hand. Positive: >3-5 mm
                                               translation and a soft/mushy end-point.

   Pivot Shift Test     ACL                    Patient supine; examiner applies internal tibial
                        (Rotational            rotation and valgus stress while slowly flexing
                        Instability)           the knee from full extension. Positive: subluxated
                                               lateral tibial plateau abruptly reduces with a
                                               palpable "clunk" at 20°-30° flexion (Spec: >98%).

   Posterior Drawer     PCL                    Knee flexed 90°, hips 45°; examiner pushes
   Test                 (Gold Standard)        proximal tibia posteriorly. Positive: posterior
                                               displacement >5 mm (Sens: ~90%, Spec: 99%).

   Posterior Sag        PCL                    Hips and knees flexed 90° with heels supported.
   Sign (Godfrey)                              Gravity causes the proximal tibia to sag posteriorly
                                               relative to the femoral condyles.

   Quadriceps           PCL                    From 90° flexed sag position, patient actively
   Active Test                                 contracts quadriceps. Patellar tendon pulls the
                                               sagging tibia anteriorly back to neutral alignment.

   Valgus Stress        MCL                    Valgus force applied at 30° flexion (isolates MCL)
   Test                                        and at 0° full extension (evaluates MCL +
                                               posterior oblique ligament + cruciate ligaments).

   Varus Stress         LCL & PLC              Varus force applied at 30° flexion (isolates LCL)
   Test                                        and at 0° extension (evaluates LCL + PLC +
                                               cruciates). Laxity at 0° indicates major disruption.
   ════════════════════════════════════════════════════════════════════════════════════════

Grading Collateral Ligament Sprains

• Grade I (Mild): Microscopic interstitial stretching without macroscopic fiber disruption. Local tenderness over the femoral origin or tibial insertion; 0 to 5 mm of joint line opening with a crisp, solid end-point on stress testing at 30°.
• Grade II (Moderate): Incomplete, partial tear of ligament fibers. Moderate localized edema, ecchymosis, and pain; 5 to 10 mm of joint line opening with a firm but distinct end-point.
• Grade III (Severe): Complete structural disruption of the ligament. Severe pain initially (may paradoxically lessen after complete rupture); >10 mm of joint line opening with an absent, soft, or indeterminate end-point. If valgus or varus laxity is present in full 0° knee extension, a combined cruciate or posterior capsular disruption is present by definition.

Segond Fracture: Pathognomonic Radiographic Sign

A Segond fracture is a small, vertical avulsion fracture off the cortical margin of the anterolateral tibial plateau, immediately distal to the joint line, occurring at the insertion of the anterolateral ligament (ALL). While present in only 10% to 15% of acute knee injuries, its presence on an anteroposterior radiograph is pathognomonic for an ACL rupture in 75% to 100% of cases, frequently accompanied by lateral meniscal tears.

The Unhappy Triad of O'Donoghue

Originally described by Fred O'Donoghue in 1950, the classical "Unhappy Triad" results from high-velocity lateral contact with the foot fixed, leading to severe valgus stress, knee flexion, and external tibial rotation. It comprises:

1. Anterior Cruciate Ligament (ACL) tear;
2. Medial Collateral Ligament (MCL) tear; and
3. Medial Meniscus tear (Note: Modern MRI and biomechanical investigations demonstrate that in acute non-contact injuries, lateral meniscal tears actually occur more frequently; however, classic board examination questions continue to test the historical triad featuring the medial meniscus).

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Meniscal Tears & Vascular Zoning

The medial and lateral menisci are crescentic fibrocartilaginous structures composed predominantly of type I collagen arranged in circumferential hoop fibers (absorbing axial compressive forces) and radial tie fibers (preventing longitudinal splitting).

Medial vs. Lateral Meniscal Anatomy

• Medial Meniscus: Semicircular (C-shaped), larger diameter, firmly anchored to the joint capsule and deep MCL. It has limited mobility (translates <5 mm during flexion-extension). Because of this rigid static fixation, the medial meniscus is torn two to three times more frequently than the lateral meniscus during traumatic rotational forces.
• Lateral Meniscus: More circular (O-shaped), smaller diameter, covers up to 70% of the lateral tibial plateau. It is unattached to the LCL, interrupted posterolaterally by the popliteus tendon hiatus, and displays substantial mobility (translating 9 to 11 mm during movement), conferring greater resilience against shear stresses.

Meniscal Vascular Zones & Healing Potential

In 1982, Arnoczky and Warren established the microvascular architecture of the adult meniscus, which dictates clinical management:

                         MENISCAL VASCULAR ZONING

     [Femoral Condyle / Synovial Periphery]
     ═══════════════════════════════════════════════════════════════════════
      RED-RED ZONE (Outer 10% - 30%)
      • Rich perimeniscal capillary plexus from genicular arteries
      • High intrinsic healing capacity
      • Treatment: Arthroscopic Meniscal Suture Repair
     ───────────────────────────────────────────────────────────────────────
      RED-WHITE ZONE (Middle 30%)
      • Intermediate vascular arborization
      • Variable healing capacity based on tear geometry and patient age
     ───────────────────────────────────────────────────────────────────────
      WHITE-WHITE ZONE (Inner 60% - 70%)
      • Completely avascular; nourished strictly by synovial fluid diffusion
      • ZERO intrinsic healing capacity
      • Treatment: Conservative therapy or Partial Meniscectomy (Resection)
     ═══════════════════════════════════════════════════════════════════════
     [Central Free Edge / Intercondylar Notch]

Clinical Presentation & Provocative Examination

• Mechanism: Traumatic rotational shear on a planted, flexed knee in young athletes; or low-energy squatting, pivoting, or rising from a low chair in older adults with pre-existing degenerative meniscal fraying.
• Temporal Effusion Pattern: Unlike the immediate hemarthrosis of ACL disruption, meniscal tears produce a delayed, insidious joint effusion developing over 24 to 48 hours. This effusion represents reactive synovial fluid secretion provoked by intra-articular tissue irritation.
• Mechanical Symptoms: Patients describe intermittent joint line catching, clicking, or true mechanical locking (an inability to passively or actively achieve terminal knee extension due to a displaced bucket-handle meniscal fragment trapped within the intercondylar notch).
• Provocative Examination Maneuvers:
  1. Joint Line Tenderness: Palpation along the medial and lateral joint lines with the knee flexed 90°. Possesses a sensitivity of 75% to 85%, making it the single most reliable clinical screening finding.
  2. McMurray Test: Patient supine, knee fully flexed. The examiner places one hand along the joint line while the other grasps the heel. To test the medial meniscus, the examiner externally rotates the tibia, applies a valgus stress, and smoothly extends the knee. To test the lateral meniscus, the examiner internally rotates the tibia, applies a varus stress, and extends the knee. A positive test is the elicitation of a painful, palpable click, pop, or snap along the joint line.
  3. Thessaly Test (Dynamic Meniscal Test): The patient stands on the affected leg with the knee flexed to 20° while holding the examiner's outstretched hands for balance. The patient then actively rotates the femur on the fixed tibia internally and externally three times. Elicitation of joint line discomfort or a mechanical catching sensation achieves a diagnostic accuracy exceeding 90% (superior to McMurray and Apley tests).
  4. Apley Compression & Distraction Test: Patient prone with knee flexed to 90°. Downward axial compression and rotation reproducing pain suggests meniscal pathology; upward distraction and rotation reproducing pain suggests collateral ligament strain.

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Patellofemoral Pain Syndrome (PFPS / "Runner's Knee")

Patellofemoral pain syndrome is the single most common cause of anterior knee pain evaluated in primary care, accounting for 25% to 40% of all running-related clinical visits, with a strong female predilection (2:1 female-to-male ratio).

Pathophysiology & Biomechanics

• Lateral Patellar Maltracking: During normal knee kinematics, the patella enters the femoral trochlear groove at approximately 20° of knee flexion and tracks smoothly within the central sulcus. In PFPS, abnormal lateral vector forces cause the patella to tilt and subluxate laterally, resulting in focal elevation of patellofemoral contact pressures, subchondral bone stress, and richly innervated lateral retinacular strain.
• Dynamic Knee Valgus: The underlying driver is frequently proximal or distal kinetic chain failure rather than an intrinsic knee disorder:
  • Proximal Kinetic Chain: Weakness and delayed activation of the hip abductors (gluteus medius) and hip external rotators (gluteus maximus) allow the femur to adduct and internally rotate during weight-bearing stance, driving the knee into dynamic valgus.
  • Local Extensor Imbalance: Atrophy or delayed neuromuscular firing of the vastus medialis obliquus (VMO) relative to the dominant vastus lateralis fails to provide medial counter-traction.
  • Distal Kinetic Chain: Excessive subtalar joint pronation induces compensatory internal tibial rotation, further accentuating the Q-angle (quadriceps angle formed between the vector of the rectus femoris and the line of the patellar tendon; normal is 10° to 15° in males, 15° to 18° in females).

Clinical Presentation & Provocative Examination

• Symptom Complex: Insidious onset of diffuse, dull, aching anterior retro-patellar or peripatellar pain. Pain is characteristically exacerbated by activities that dramatically increase patellofemoral joint reaction forces (which reach 3 times body weight during stair ambulation and up to 7 to 8 times body weight during deep squats):
  • Descending stairs (consistently more painful than ascending due to eccentric quadriceps loading);
  • Deep squatting, lunging, running, and kneeling;
  • Prolonged sitting with knees maintained in flexion (e.g., in an automobile, airplane, or movie theater), termed the "movie theater sign" or "theater sign", caused by continuous sustained contact pressure between the patellar articular facets and the femoral trochlea.
• Physical Examination Findings:
  • Clarke Test (Patellar Grind Test): Patient supine with knee extended. Examiner applies gentle downward and posterior pressure on the superior pole of the patella while instructing the patient to contract the quadriceps. Reproduction of retro-patellar pain and inability to maintain contraction is positive (caveat: high sensitivity but moderate specificity; often uncomfortable even in asymptomatic controls).
  • Patellar Tilt & Glide Test: Assesses tightness of the lateral retinaculum. Inability to tilt the lateral patellar facet up to horizontal indicates severe lateral retinacular tethering.
  • J-Sign: As the knee moves from 90° flexion into full extension, the patella tracks smoothly in the trochlear groove until terminal extension (0° to 10°), where it abruptly jerks laterally into an inverted "J" trajectory due to unopposed vastus lateralis pull.
  • Functional Dynamic Assessment: Performance of a single-leg mini-squat demonstrates dynamic knee valgus collapse, pelvic tilt (contralateral drop), and reproduction of anterior knee aching.

Evidence-Based Management Protocol

Conservative non-operative management is universally recognized as first-line therapy, achieving lasting symptom resolution in >85% to 90% of patients:

1. Kinetic Chain Physical Therapy (Cornerstone of Management):
   • Closed-Chain Quadriceps Strengthening: Mini-squats (0° to 45°), leg presses, and step-ups. Closed kinetic chain exercises distribute joint contact forces across larger articular surface areas, avoiding the deleterious peak compressive forces generated during open-chain terminal knee extension (0° to 30° leg extensions on machines, which are strictly contraindicated).
   • Proximal Hip Stabilization: Targeted strengthening of the gluteus medius, gluteus maximus, and core stabilizers (side-lying clam shells, lateral band walks, single-leg bridges) to eliminate dynamic valgus collapse.
   • Soft-Tissue Flexibility: Passive stretching of the iliotibial band, hamstrings, and gastrocnemius-soleus complex.
2. Adjunctive Modalities:
   • Patellar Taping (McConnell Taping): Application of rigid adhesive tape to provide medial glide and tilt offloads the lateral retinaculum, providing immediate short-term pain relief (2 to 4 weeks) to facilitate exercise compliance.
   • Orthotic Insoles: Prefabricated arch supports for individuals with excessive foot pronation.
   • Pharmacotherapy: Short-term oral NSAIDs for acute inflammatory flares.
   • Surgical Warning: Surgical interventions (e.g., isolated arthroscopic lateral retinacular release) have poor evidence in isolated PFPS without objective patellar dislocation, and can trigger iatrogenic medial patellar instability.

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Osgood-Schlatter Disease (Adolescent Traction Apophysitis)

Osgood-Schlatter disease is an aseptic traction apophysitis of the tibial tuberosity resulting from repetitive tensile microtrauma delivered by the extensor mechanism to the secondary ossification center of the tibial apophysis during the rapid adolescent growth spurt.

• Epidemiology: Occurs predominantly in athletically active adolescents participating in running, jumping, and kicking sports (basketball, soccer, gymnastics). Classically affects boys aged 12 to 15 years and girls aged 10 to 13 years, corresponding to peak skeletal growth velocity; bilateral in 25% to 30% of cases.
• Pathophysiology: Bone elongation during adolescent growth outpaces the lengthening capacity of the quadriceps muscle-tendon unit, creating high passive tendon tension. Repetitive forceful quadriceps contractions during jumping and sprinting pull avulsion microfractures at the cartilaginous attachment of the patellar tendon into the secondary tibial ossification center. The resulting reparative cascade causes localized fibrous hyperplasia, soft tissue swelling, and heterotopic ossification.
• Clinical Presentation: Insidious development of localized anterior knee pain, accompanied by an enlarging, prominent, exquisitely tender bony bump over the tibial tuberosity. Pain is worsened by running, jumping, stair climbing, and direct contact (kneeling), and is relieved by rest.
• Physical Examination: Marked point tenderness directly over the anterior tibial tubercle; localized soft tissue edema and bony prominence; reproduction of pain on resisted active knee extension from 90° flexion or during passive extreme knee flexion (stretching the extensor mechanism).
• Diagnostic Imaging: Diagnosis is strictly clinical. Plain lateral knee radiographs are not routinely required unless red flags are present (systemic symptoms, pain at rest, night waking, or atypical unilateral swelling to exclude osteosarcoma or complete tibial tubercle avulsion fracture). Radiographs characteristically demonstrate soft tissue swelling and irregular ossification, fragmentation, or separation of the tibial tubercle apophysis.
• Management & Prognosis:
  • Natural History: Self-limiting and universally benign; spontaneous complete resolution occurs upon skeletal maturity when the tibial growth plate closes (around age 14 in females and 16 in males).
  • Conservative Treatment: Activity modification based on symptom tolerance (relative rest without total cessation of sports), post-activity ice application, quadriceps and hamstring stretching, and short-term acetaminophen or ibuprofen. A patellar tendon counterforce strap (infrapatellar strap) placed over the tendon mid-substance disperses tensile loading away from the tibial insertion.
  • Patient Reassurance: Parents and adolescents should be counseled that a prominent, painless bony knob over the tibial tuberosity frequently persists into adulthood without causing functional limitation.

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Clinical Decision Rules: The Ottawa Knee Rules

To safely rule out knee fractures and reduce unnecessary, low-yield diagnostic radiographs in acute knee trauma, Stiell and colleagues developed and validated the Ottawa Knee Rules in emergency departments and primary care settings.

                         THE OTTAWA KNEE RULES

   A knee radiograph series (AP, Lateral, Sunrise) is indicated ONLY if the
   patient has sustained acute knee trauma AND presents with ANY of the following:

   ┌───┬───────────────────────────────────────────────────────────────────┐
   │ 1 │ Age 55 years or older                                             │
   ├───┼───────────────────────────────────────────────────────────────────┤
   │ 2 │ Isolated tenderness of the patella (with no other bony tenderness)│
   ├───┼───────────────────────────────────────────────────────────────────┤
   │ 3 │ Tenderness at the head of the fibula                              │
   ├───┼───────────────────────────────────────────────────────────────────┤
   │ 4 │ Inability to flex the knee to 90 degrees                          │
   ├───┼───────────────────────────────────────────────────────────────────┤
   │ 5 │ Inability to bear weight BOTH immediately following the trauma    │
   │   │ AND in the clinic/ED (defined as inability to take 4 steps;       │
   │   │ limping counts as bearing weight)                                 │
   └───┴───────────────────────────────────────────────────────────────────┘

• Diagnostic Accuracy: The Ottawa Knee Rules demonstrate a pooled sensitivity approaching 100% (98.5% to 100%) for detecting clinically significant fractures (patellar, tibial plateau, fibular, distal femoral fractures), with a negative predictive value >99.7%. Application of the rules reduces knee radiography utilization by 28% to 35% without missing significant fractures.
• Pittsburgh Knee Rules Alternative: Fall or blunt trauma mechanism PLUS either: age <12 or >50 years, OR inability to walk four weight-bearing steps in the clinic/ED (equally sensitive, slightly more specific in children).

Indications for Knee Magnetic Resonance Imaging (MRI)

While plain radiographs rule out fractures, non-contrast MRI is the definitive modality for evaluating intra-articular soft-tissue disruptions:

1. Acute Locked Knee: Displaced bucket-handle meniscal tear or detached osteochondral loose body physically blocking joint extension; requires urgent MRI and expedited arthroscopic decompression.
2. Suspected Cruciate or Multiligamentous Disruption: In high-demand active individuals or competitive athletes being evaluated for surgical ligamentous reconstruction.
3. Suspected Knee Dislocation: Spontaneously reduced knee dislocations (involving two or more major ligaments) require urgent MRI and immediate emergency vascular assessment (measuring the Ankle-Brachial Index [ABI]; an ABI <0.9 mandates emergent CT angiography to exclude occult popliteal artery intimal tear or transection).
4. Refractory Symptoms: Persistent joint line pain, mechanical catching, or unresolving effusion despite 4 to 6 weeks of structured conservative therapy.