10.2 Knee Conditions: Ligamentous Tears, Meniscal Injury & Patellofemoral Pain
Key Takeaways
- The screw-home mechanism locks the knee in terminal extension (tibia externally rotates 5°–10° on the fixed femur in open chain); the popliteus muscle unlocks the knee by internally rotating the tibia in open chain or externally rotating the femur in closed chain.
- Anterior Cruciate Ligament (ACL) physical examination relies on the Lachman test (highest sensitivity, 85%–95%) and pivot shift test (highest specificity, >98%); early reconstruction priorities mandate immediate restoration of full passive knee extension (0°) and quadriceps activation without lag.
- During early ACL rehabilitation, open-kinetic chain (OKC) seated knee extension is restricted to 90°–45° of flexion because 40°–0° produces peak anterior tibial shear and excessive graft strain; closed-kinetic chain (CKC) squats from 0°–60° are safe due to joint compression and hamstring co-contraction.
- Meniscal tear management is dictated by vascularity: red-red peripheral tears have high healing capacity and are repaired, requiring post-op flexion restricted to ≤90° for 4–6 weeks and avoiding loaded squats >90° for 3–4 months; white-white avascular tears undergo partial meniscectomy with immediate WBAT and early unrestricted ROM.
- Post-Total Knee Arthroplasty (TKA) rehabilitation strictly prohibits the continuous use of a pillow placed directly beneath the operative knee in bed to prevent development of a fixed knee flexion contracture; functional ROM targets are 0° extension, ≥90° flexion by week 2, and 110°–120° for stair descent.
10.2 Knee Conditions: Ligamentous Tears, Meniscal Injury & Patellofemoral Pain
[!NOTE] DHA Clinical Competency Focus: Knee joint pathomechanics and post-surgical rehabilitation represent foundational topics for the DHA Physiotherapist licensing assessment. Candidates must understand the exact biomechanical drivers of the screw-home mechanism, compare diagnostic accuracy metrics for ACL and meniscal tests, differentiate safe versus hazardous loading arcs in ACL reconstruction rehabilitation, establish precise weight-bearing and ROM restrictions for meniscal repairs versus partial meniscectomy, address proximal kinetic chain drivers in patellofemoral pain syndrome, and enforce critical nursing and positioning precautions following Total Knee Arthroplasty (TKA).
The knee is a modified bicondylar synovial hinge joint operating at the intersection of the body's two longest lever arms (the femur and tibia). It relies primarily on soft tissue restraints—the cruciate ligaments, collateral ligaments, fibrocartilaginous menisci, and dynamic musculotendinous units—for dynamic stability.
1. Knee Arthrokinematics, the Screw-Home Mechanism & Patellofemoral Alignment
The Screw-Home Mechanism
The screw-home mechanism describes the obligatory conjunct axial rotation that occurs during the final 20° to 30° of knee extension, locking the knee into a mechanically stable, energy-efficient close-packed state.
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| The Knee Screw-Home Mechanism |
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| Terminal Extension (Locking): |
| - Open Kinetic Chain (OKC): Tibia EXTERNALLY rotates 5° to 10° on fixed femur |
| - Closed Kinetic Chain (CKC): Femur INTERNALLY rotates on fixed tibia |
| - Drivers: Larger medial femoral condyle articular surface, passive ACL tension,|
| slight lateral vector of the quadriceps muscle pull |
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| Flexion Initiation (Unlocking): |
| - Unlocked by: POPLITEUS muscle |
| - Open Kinetic Chain (OKC): Popliteus INTERNALLY rotates tibia on femur |
| - Closed Kinetic Chain (CKC): Popliteus EXTERNALLY rotates femur on tibia |
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- Biomechanical Drivers of Locking:
- Asymmetrical Condylar Geometry: The articular surface of the medial femoral condyle extends further anteriorly and has a larger surface area than the lateral femoral condyle. As the lateral condyle completes its rolling-gliding excursion early in terminal extension, the medial condyle continues to glide posteriorly, obligating tibial external rotation in open chain.
- Passive Ligamentous Tension: The anterior cruciate ligament (ACL) becomes taut in terminal extension, guiding the lateral compartment into passive external rotation.
- Lateral Quadriceps Vector: The slight lateral line of pull of the quadriceps creates an external rotational moment on the tibia via the patellar tendon.
- The Popliteus Muscle: Key to the Knee:
- Innervated by the tibial nerve (L4–S1), the popliteus originates on the lateral epicondyle of the femur (intra-capsular, extra-synovial) and inserts onto the posterior proximal surface of the tibia above the soleal line.
- To initiate knee flexion from full extension, the popliteus must contract to unlock the joint. In open-chain movement, it acts as a primary internal rotator of the tibia. In closed-chain weight-bearing movement, it externally rotates the femur on the stable tibia.
Patellofemoral Mechanics & The Q-Angle
- Definition of Q-Angle (Quadriceps Angle): The angle formed between: (1) A line connecting the Anterior Superior Iliac Spine (ASIS) to the midpoint of the patella; and (2) A line connecting the midpoint of the patella to the tibial tuberosity.
- Normal Reference Values:
- Males: 10° to 14°
- Females: 15° to 18° (due to a broader anatomical pelvis and wider pelvic width-to-femur length ratio)
- Clinical Significance: A Q-angle exceeding 20° significantly increases the lateral "bowstringing" vector acting on the patella during quadriceps contraction. This increases lateral patellofemoral contact pressures, accelerates lateral facet chondromalacia, and substantially elevates the risk of lateral patellar subluxation or dislocation.
2. Anterior Cruciate Ligament (ACL) Injury & Reconstruction Rehabilitation
Injury Mechanism & Diagnostic Special Tests
- Mechanism: Over 70% of ACL ruptures are non-contact events occurring during rapid deceleration, jump landing, or sidestep cutting maneuvers combining knee flexion, dynamic valgus collapse, and tibial internal or external rotation. Patients report a distinct internal "pop", immediate swelling (hemarthrosis within 2–6 hours), and acute instability.
| Diagnostic Test | Execution Technique | Diagnostic Accuracy & Role |
|---|---|---|
| Lachman Test | Knee flexed 20°–30°, patient supine. Examiner stabilizes distal femur and applies rapid anterior translation to proximal tibia. | Highest Sensitivity (85%–95%), Specificity ~95%. Primary test to rule out and rule in ACL tear. Evaluates anterior translation distance and endpoint quality (soft/mushy endpoint = tear). |
| Anterior Drawer Test | Knee flexed 90°, foot stabilized on table. Examiner translates proximal tibia anteriorly. | Sensitivity lower in acute phase (50%–65%) due to hemarthrosis, joint effusion, and reflexive hamstring spasm blocking motion; high specificity (~90%). |
| Pivot Shift Test | Lower limb brought from extension to 30°–40° flexion while applying internal rotation and valgus stress. | Highest Specificity (>98%), low sensitivity (30%–40% awake due to muscle guarding; >85% under anesthesia). Assesses anterolateral rotatory instability; positive when tibia subluxes anteriorly in extension and visibly reduces at ~30° flexion with a palpable clunk. |
Graft Options & Biomechanical Donor Morbidity
- Bone-Patellar Tendon-Bone (BPTB) Autograft: Central third of patellar tendon with bone plugs from patella and tibial tuberosity. Gold standard for rigid bone-to-bone fixation (heals in 6–8 weeks). Morbidities: Persistent anterior knee pain, patellar tendinitis, pain with kneeling, and risk of patellar fracture.
- Hamstring Tendon (Semitendinosus/Gracilis - STG) Autograft: Quadrupled strand. Advantages: Smaller incision, negligible kneeling pain. Morbidities: Soft tissue-to-bone healing takes 8–12 weeks; residual donor hamstring weakness (especially deep flexion and internal rotation strength).
- Quadriceps Tendon Autograft: Thick collagenous profile; lower anterior knee pain than BPTB, but requires strict quadriceps activation retraining post-op.
Biomechanical Constraints & Safe Loading Arcs
During graft remodeling (ligamentization), excessive shear forces can stretch or tear the healing graft. Physical therapists must enforce strict open- versus closed-kinetic chain parameters:
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| ACL Reconstruction: Safe vs Hazardous Loading Arcs |
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| Open-Kinetic Chain (OKC) Knee Extension (e.g., seated leg extension machine): |
| - Hazardous Arc: 40° to 0° Extension -> Produces PEAK anterior tibial shear, |
| maximum patellar tendon vector angle, and highest ACL graft strain. PROHIBITED|
| in early/intermediate phases! |
| - Safe Arc: 90° to 45° Flexion -> Anterior shear is minimal or neutralized|
| by posterior capsular structures. Permitted for early isolated quad loading. |
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| Closed-Kinetic Chain (CKC) Weight-Bearing (e.g., leg press, squats): |
| - Safe Arc: 0° to 60° Knee Flexion -> Compressive axial joint loading and |
| synergistic hamstring co-contraction pull tibia posteriorly, protecting graft.|
| - Caution: Deep squats (>90°) increase patellofemoral and posterior shear forces|
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- Early Phase Priorities (Weeks 0–2):
- Immediate Full Passive Knee Extension (0°): Equal to contralateral hyperextension. Failure to achieve full extension within 2 weeks leads to permanent arthrofibrosis, altered gait mechanics, and elevated patellofemoral pain.
- Quadriceps Activation Without Lag: Eliminate extensor lag on Straight Leg Raise (SLR) using neuromuscular electrical stimulation (NMES), superior patellar glides, and active quadriceps isometric sets.
- Swelling Control: Cryotherapy, compression, and elevation to abolish arthrogenic muscle inhibition (AMI).
3. Meniscal Injuries: Vascular Zones, Diagnostic Clustering & Rehabilitation Divergence
Vascularity & Healing Capacity (Arnoczky & Warren Zones)
- Red-Red Zone (Outer 10%–30%): Fully vascularized by the perimeniscal capillary plexus arising from genicular arteries. Tears in this zone possess excellent biological healing capacity and are prime candidates for surgical meniscal repair.
- Red-White Zone (Middle Third): Borderline vascularity; variable healing capacity.
- White-White Zone (Inner 60%–70%): Completely avascular; receives nutrition exclusively through synovial fluid diffusion. Tears in this zone cannot heal biologically and are treated with partial meniscectomy (surgical debridement).
[ Femoral Condyle ]
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▼
┌─────────────────────────────────┐
│ Red-Red (Outer 10-30%) │ ◄── Rich blood supply -> REPAIR CANDIDATE
├─────────────────────────────────┤
│ Red-White (Middle) │ ◄── Intermediate / marginal vascularity
├─────────────────────────────────┤
│ White-White (Inner 60-70%) │ ◄── Completely avascular -> MENISCECTOMY
└─────────────────────────────────┘
▲
│
[ Tibial Plateau ]
Special Diagnostic Tests for Meniscal Tears
- Joint Line Tenderness (JLT): Palpation along joint line; high sensitivity (~75%–85%), moderate specificity.
- McMurray Test: Patient supine, knee fully flexed. External rotation + valgus stress while extending knee tests the medial meniscus; Internal rotation + varus stress tests the lateral meniscus. Positive: Palpable click, audible snap, or localized pain.
- Apley Compression & Distraction Test: Patient prone, knee flexed 90°. Examiner applies axial downward compression with rotation (provokes meniscus). Examiner then applies upward distraction with rotation (relieves meniscus; pain indicates ligamentous injury).
- Thessaly Test (at 20° Flexion): Patient stands on test leg flexed to 20° and rotates the body internally and externally three times. Highest overall diagnostic accuracy (sensitivity 90%, specificity 96%).
Rehabilitation Divergence: Partial Meniscectomy vs. Meniscal Repair
The physical therapy protocols for partial meniscectomy versus meniscal repair are diametrically opposed, representing a major DHA exam testing point:
| Parameter | Partial Meniscectomy | Meniscal Repair (Suture Fixation) |
|---|---|---|
| Pathology / Zone | Avascular white-white zone tear | Vascular red-red zone tear |
| Initial Weight-Bearing | Immediate WBAT without brace; progress to full weight-bearing within days as tolerated. | Protected Weight-Bearing (PWB or NWB); often non-weight-bearing or touch-down weight-bearing with hinged brace locked at 0° for 2–4 weeks. |
| ROM Restrictions | Immediate active and passive ROM as tolerated (no restrictions). | Strict flexion limitation: ≤90° for 4 to 6 weeks to prevent posterior horn extrusion and suture tear. |
| Squatting / Loading | Light squats as tolerated by swelling and comfort. | Deep loaded squats (>90° flexion) strictly prohibited for 12 to 16 weeks (3–4 months). |
| Return to Sport | Rapid (typically 4 to 6 weeks). | Prolonged (typically 5 to 6 months) to permit biological fibrocartilage healing. |
4. Collateral Ligaments & Patellofemoral Pain Syndrome (PFPS)
Collateral Ligament Evaluation
- Medial Collateral Ligament (MCL): Stressed by valgus force.
- Valgus Stress Test at 30° Flexion: Isolates the superficial MCL (capsular restraints relaxed).
- Valgus Stress Test at 0° Full Extension: A positive test with gross opening at 0° indicates a catastrophic multi-ligamentous injury involving the MCL, posteromedial capsule, and ACL/PCL.
- Management: Vast majority of Grade I–III isolated MCL sprains heal conservatively with hinged bracing.
- Lateral Collateral Ligament (LCL): Stressed by varus force. Tested with Varus Stress Test at 30° and 0°. Isolated tears are rare; rule out posterolateral corner (PLC) injuries.
Patellofemoral Pain Syndrome (PFPS)
- Pathophysiology: Retropatellar or peripatellar pain exacerbated by activities that increase patellofemoral joint reaction forces (squatting, stair climbing, prolonged sitting with knees flexed ["theater sign"]). Driven by patellar maltracking within the femoral trochlea.
- Proximal & Distal Kinetic Chain Factors:
- Proximal Driver: Weakness in hip abductors (gluteus medius) and external rotators causes the femur to adduct and internally rotate under the patella during dynamic tasks ("the femur moves under the patella, rather than the patella moving on the femur"), precipitating dynamic knee valgus.
- Distal Driver: Excessive foot pronation / subtalar eversion causes compensatory internal tibial rotation, increasing the functional Q-angle.
- Evidence-Based Interventions:
- Combined proximal (hip abductor/gluteal) and local (quadriceps/VMO) strengthening is significantly superior to quadriceps strengthening alone.
- McConnell Taping: Rigid tape applied from lateral to medial across the patella to induce a medial patellar glide, immediately reducing lateral facet contact stress and allowing pain-free quadriceps loading.
5. Total Knee Arthroplasty (TKA): Milestones & Flexion Contracture Prevention
Total Knee Arthroplasty replaces damaged articular surfaces with femoral, tibial, and frequently patellar prosthetic components.
Functional ROM Milestones
- 0° Extension: Essential for normal stance phase knee extension and energy-efficient gait.
- ≥90° Flexion by Week 2: Critical benchmark for independent mobility.
- 105°–110° Flexion: Required for reciprocal stair descent.
- 115°–120° Flexion: Required for sitting and rising effortlessly from low chairs or floor surfaces.
THE CARDINAL POST-TKA RULE: Flexion Contracture Prevention
[!CAUTION] Prohibition of Pillow Placement Under the Knee: In the inpatient and home health setting, patients often request a pillow placed beneath their operative knee because slight flexion (~30°) relieves intra-capsular pressure and reduces pain. Physiotherapists must strictly forbid continuous pillow placement directly under the popliteal fossa / knee! Sustained knee flexion promotes rapid shortening of the posterior capsule, hamstrings, and gastrocnemius, cementing a fixed knee flexion contracture. An extension deficit as small as 5°–10° causes an abnormal gait, quadriceps fatigue, and back/hip pain. Pillows or towel rolls must be placed strictly beneath the heel or Achilles tendon, allowing gravity to assist passive terminal extension.
6. Clinical Scenario & DHA Exam Traps
Clinical Scenario: ACL Reconstruction Rehabilitation Prescription
Scenario: A 24-year-old female soccer player is 5 weeks post-operative following right ACL reconstruction utilizing a bone-patellar tendon-bone (BPTB) autograft. She presents to the clinic with minimal effusion, full passive extension (0°), and active knee flexion to 115°. Her quad lag is resolved. The junior physical therapist proposes initiating open-chain seated knee extension on a weight machine from 90° to full extension (0°) with a 10 kg resistance to accelerate quadriceps hypertrophy.
Clinical Appraisal & Intervention: The proposed prescription is contraindicated and hazardous. Open-kinetic chain (OKC) knee extension between 40° and 0° produces maximum anterior tibial shear force, excessively stressing the immature BPTB graft during its vulnerable ligamentization remodeling phase. The supervising physiotherapist must modify the program:
- Restrict any open-chain seated quadriceps extension to the safe arc of 90° to 45° of flexion.
- Prioritize closed-kinetic chain exercises (such as leg press, mini-squats, and step-ups) between 0° and 60° of knee flexion, where hamstring co-contraction and axial joint compression neutralize anterior shear forces.
DHA Exam Traps to Avoid
[!WARNING]
- Trap 1: Open vs Closed Chain ACL Arcs: Do not confuse safe angles for OKC versus CKC. In open chain (seated leg extension), 40° to 0° is dangerous; 90° to 45° is safe. In closed chain (squats/leg press), 0° to 60° is safe; deep squats (>90°) increase patellofemoral and posterior shear stress.
- Trap 2: Meniscal Repair vs. Meniscectomy Restrictions: When a vignette describes a patient post-meniscal repair, never select immediate full weight-bearing or aggressive deep squats. Meniscal repair mandates protected weight-bearing, flexion limited to ≤90° for 4–6 weeks, and avoiding deep squats (>90°) for 3–4 months. Partial meniscectomy allows immediate WBAT and unrestricted ROM.
- Trap 3: Unlocking the Screw-Home Mechanism: DHA questions regularly ask which muscle unlocks the fully extended knee. The correct answer is the popliteus. Remember that it internally rotates the tibia in open chain, but externally rotates the femur in closed chain.
A physiotherapist designs a rehabilitation program for a 21-year-old athlete who is 6 weeks post-operative following an ACL reconstruction with a bone-patellar tendon-bone autograft. To protect the healing graft from excessive anterior tibial shear strain while strengthening the quadriceps, how should exercise loading arcs be prescribed?
A 28-year-old professional basketball player undergoes arthroscopic repair of a bucket-handle tear in the peripheral red-red vascular zone of the medial meniscus. Which postoperative rehabilitation guideline represents the standard evidence-based protocol during the initial 4 to 6 weeks?
During the physical examination of a patient with an extension deficit of the knee, the physiotherapist analyzes the arthrokinematics of the screw-home mechanism. Which of the following correctly describes the kinematic events occurring during terminal extension and subsequent flexion initiation?