Classification
- Type: Modified synovial hinge (with rotation component)
- Degrees of freedom: 2 (flexion/extension as the primary motion; axial rotation available when the knee is flexed)
- Region: Knee complex (along with the patellofemoral and proximal tibiofibular articulations)
Articular Surfaces
- Femoral condyles (convex): Two large condyles — medial and lateral — covered with thick hyaline cartilage. The medial condyle is larger, extends further distally, and has a greater anterior-posterior curvature. This asymmetry produces the "screw-home mechanism" (see Clinical Notes). The intercondylar notch separates the condyles posteriorly and houses the cruciate ligaments.
- Tibial plateau (concave): Two relatively flat surfaces — medial and lateral tibial plateaus — separated by the intercondylar eminence. The medial plateau is slightly concave (dish-shaped); the lateral plateau is slightly convex (which makes it more mobile and more likely to sublux). Without the menisci, the tibial surfaces would provide poor congruence with the curved femoral condyles.
- Menisci: Two C-shaped fibrocartilaginous discs that sit on the tibial plateaus. The medial meniscus is larger, more C-shaped, and firmly attached to the joint capsule and MCL — making it less mobile and more vulnerable to injury (3:1 medial-to-lateral tear ratio). The lateral meniscus is smaller, more circular, and less firmly attached — making it more mobile. Together, the menisci deepen the tibial articular surfaces, distribute load across a wider area (reduce contact stress by ~50%), provide shock absorption, improve lubrication, and contribute to proprioception.
- Cartilage: Hyaline cartilage on the femoral condyles and tibial plateaus. The femoral cartilage is thickest centrally (up to 6 mm) — the thickest cartilage in the body — reflecting the enormous compressive forces this joint sustains (up to 3–4 times body weight during walking, 7–8 times during stair descent).
Movements and ROM
| Movement | Normal ROM | Plane | Muscles Producing |
|---|---|---|---|
| Flexion | 130–150° | Sagittal | anatomy/muscles/hamstrings (biceps femoris, semimembranosus, semitendinosus), anatomy/muscles/gastrocnemius, anatomy/muscles/popliteus, anatomy/muscles/sartorius, anatomy/muscles/gracilis |
| Extension | 0° (to 10–15° hyperextension) | Sagittal | anatomy/muscles/quadriceps (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) |
| Internal rotation (in flexion) | 20–30° | Transverse | anatomy/muscles/popliteus (primary IR of tibia), anatomy/muscles/semitendinosus, anatomy/muscles/semimembranosus, anatomy/muscles/sartorius, anatomy/muscles/gracilis |
| External rotation (in flexion) | 30–40° | Transverse | anatomy/muscles/biceps-femoris, anatomy/muscles/tensor-fasciae-latae / ITB |
Rotation is only available when the knee is flexed. In full extension, the knee is "locked" by the screw-home mechanism and the taut cruciate and collateral ligaments prevent rotation. Maximum rotational ROM occurs at approximately 90° of flexion. This has direct implications for assessment — always test rotational integrity with the knee flexed.
Capsular Pattern
Flexion > Extension When the tibiofemoral capsule is restricted (as in OA or post-immobilization), flexion loss is greater than extension loss. This distinguishes capsular pathology from ligamentous or meniscal problems, which produce non-capsular patterns. A patient who has lost 30° of flexion and only 5° of extension is following the capsular pattern — suspect OA or capsulitis.Resting Position
- 25° flexion
- Maximum joint volume, capsule and ligaments relaxed
- Use this position for joint mobilization and joint play assessment
- This is also why patients with acute knee effusion hold the knee in slight flexion — it is the position of greatest volume and least capsular pressure
Close-Packed Position
- Full extension with external rotation of the tibia (the screw-home position)
- Maximum bony congruence, all ligaments taut, menisci compressed
- Used for ligament integrity testing (collateral stress tests, drawer tests)
- Do NOT mobilize in this position
End-Feels
| Movement | Normal End-Feel | Type |
|---|---|---|
| Flexion | Tissue approximation | Soft — calf muscle bulk meets posterior thigh (in muscular individuals, this is the primary limiter). In lean individuals, a capsular (firm) end-feel may also be present. |
| Extension | Capsular / bony | Firm (capsular and ligamentous tension) progressing to hard (bony) at full hyperextension as the olecranon-like tibial eminence contacts the intercondylar notch |
| Internal rotation | Capsular (firm) | Ligamentous and capsular tension limit further rotation |
| External rotation | Capsular (firm) | Ligamentous tension (cruciate ligaments) |
Abnormal end-feels: A springy (bouncy) end-feel during extension suggests a meniscal block — torn meniscus tissue preventing full extension ("locked knee"). An empty end-feel (severe pain before any resistance) suggests acute fracture, infection, or tumor. A boggy or mushy end-feel suggests significant joint effusion.
Ligaments
Anterior Cruciate Ligament (ACL)
- Attachments: Anterior intercondylar area of the tibia → posterolateral aspect of the intercondylar notch of the femur. Runs posteriorly, laterally, and superiorly from tibia to femur.
- Function: The primary restraint against anterior tibial translation (prevents the tibia from sliding forward on the femur). Also resists IR and valgus stress. The ACL is the most critical stabilizer for pivoting, cutting, and deceleration activities.
- Injury mechanism: Non-contact deceleration with a planted foot and valgus/rotational force — the classic mechanism is landing from a jump with the knee slightly flexed and internally rotated, or cutting while running. Contact valgus injuries also damage the ACL (often as part of the "unhappy triad" with MCL and medial meniscus). Female athletes have a 3–6 times higher incidence due to wider Q-angle, narrower intercondylar notch, hormonal effects on ligament laxity, and neuromuscular factors.
- Grade classification: Grade I — microscopic tears, firm end-feel on drawer, stable; Grade II — partial tear, some laxity on drawer but firm end-feel; Grade III — complete rupture, soft or empty end-feel, significant anterior translation
- Assessment test: Anterior drawer test (knee at 90° flexion — positive: anterior tibial shift >6 mm), Lachman's test (knee at 20–30° flexion — more sensitive than anterior drawer because the hamstrings are less likely to guard; positive: soft end-feel with anterior translation), pivot shift test (identifies rotational instability — the "giving way" phenomenon)
- Condition link: conditions/acl-injury
Posterior Cruciate Ligament (PCL)
- Attachments: Posterior intercondylar area of the tibia → anterolateral aspect of the intercondylar notch of the femur (medial surface of the medial condyle). Runs anteriorly, medially, and superiorly from tibia to femur.
- Function: The primary restraint against posterior tibial translation (prevents the tibia from sliding backward on the femur). The PCL is the strongest ligament in the knee — approximately twice as strong as the ACL. It also helps control rotation and provides a central axis for knee rotation.
- Injury mechanism: Dashboard injury — a direct blow to the anterior tibia with the knee flexed (the shin hits the dashboard in a car accident, driving the tibia posteriorly). Also: falling on a flexed knee with the foot plantarflexed, or hyperflexion injuries.
- Grade classification: Same grading as ACL — I (stable), II (partial, mild laxity), III (complete, significant posterior sag)
- Assessment test: Posterior drawer test (knee at 90° flexion — positive: posterior tibial shift), posterior sag sign (with both hips and knees at 90° flexion, the injured tibia sags posteriorly compared to the uninjured side — gravity alone produces the displacement), quadriceps active test (contracting the quads in 90° flexion pulls the posteriorly-sagged tibia forward)
- Condition link: conditions/pcl-injury
Medial Collateral Ligament (MCL)
- Attachments: Medial femoral epicondyle → medial surface of the proximal tibia (approximately 6–7 cm below the joint line). The deep fibers of the MCL are attached to the medial meniscus — this is why MCL injuries often accompany medial meniscal tears.
- Function: The primary restraint against valgus (abduction) force — prevents the medial joint space from opening. Also resists ER of the tibia. The MCL is the most commonly injured knee ligament (contact sports, skiing).
- Injury mechanism: Valgus force applied to the lateral knee with the foot planted — a blow to the outside of the knee that forces the joint medially open. Classic scenario: being tackled from the outside in football or catching an edge in skiing with knee rotation.
- Grade classification: Grade I — local tenderness over MCL, stable on valgus stress at 30°, firm end-feel; Grade II — some medial opening at 30° but firm end-feel, partial tear; Grade III — significant medial opening at both 30° and 0° (full extension), soft end-feel, complete rupture. Opening at 0° extension indicates both MCL and capsular damage.
- Assessment test: Valgus stress test — applied at 30° flexion (isolates MCL by relaxing the cruciate ligaments) and at 0° extension (tests both MCL and posterior capsule). Positive: medial joint space opens with valgus force.
- Condition link: conditions/mcl-injury
Lateral Collateral Ligament (LCL)
- Attachments: Lateral femoral epicondyle → head of the fibula. Unlike the MCL, the LCL is not attached to the lateral meniscus or joint capsule — it is a cord-like structure that stands off from the joint.
- Function: The primary restraint against varus (adduction) force — prevents the lateral joint space from opening. Taut in extension, relaxed in flexion.
- Injury mechanism: Varus force applied to the medial knee (a blow to the inside of the knee that forces the joint laterally open). Less common than MCL injury because the opposite leg typically protects the medial knee. Often involves the posterolateral corner complex when damaged.
- Grade classification: Same grading as MCL. Isolated LCL injuries are less common — posterolateral corner damage often accompanies Grade III LCL tears.
- Assessment test: Varus stress test — applied at 30° flexion and 0° extension, identical procedure to valgus stress but force applied medially. Positive: lateral joint space opens.
- Condition link: Lateral collateral sprain, posterolateral corner injury
Menisci
While technically not ligaments, the menisci function as intra-articular stabilizers and are assessed alongside the ligamentous system.- Medial meniscus: Larger, C-shaped, firmly attached to the MCL and joint capsule. Less mobile → more vulnerable to injury. Tears frequently accompany MCL injuries due to their shared attachment.
- Lateral meniscus: Smaller, more circular, less firmly attached. More mobile → better able to deform under load → tears are less common but still significant.
- Function: Deepen articular surfaces, distribute compressive load, provide shock absorption, contribute to lubrication and proprioception
- Injury mechanism: Rotational force on a loaded, flexed knee — twisting on a planted foot is the classic mechanism. Medial meniscus tears in combination with ACL and MCL injuries form the "unhappy triad" (O'Donoghue's triad). Degenerative tears in older adults occur without significant trauma — the meniscus simply fails under normal loading.
- Assessment test: McMurray's test (knee flexed, tibial rotation + valgus/varus stress during extension — positive: click/pop with pain along the joint line), Apley's compression test (prone, knee at 90° — compress and rotate the tibia on the femur — pain = meniscal; distraction and rotation — pain = ligamentous), Thessaly test (weight-bearing rotation at 20° flexion)
- Condition link: conditions/meniscal-injuries
Mobilization Techniques
Hands-on instruction is required. The descriptions below provide clinical reference detail for understanding and supervised practice. They are not a substitute for instructor-led technique training. Correct hand placement, force dosage, and tissue response interpretation require hands-on coaching and feedback.
Convex-Concave Rule at the Tibiofemoral Joint
The femoral condyles are convex and the tibial plateau is concave. However, in most clinical situations, we mobilize the tibia (concave) on the fixed femur (convex) — meaning the concave-on-convex rule applies. When the concave surface moves on a fixed convex surface, the glide occurs in the same direction as the restricted movement. This is the opposite of the GH joint (where we move the convex humerus on the fixed concave glenoid). Understanding which surface you are moving is critical:| Restricted Movement | Glide Direction | Reasoning |
|---|---|---|
| Extension | Anterior tibial glide | Extension moves the tibia anteriorly on the femur → concave on convex → glide SAME direction → anterior |
| Flexion | Posterior tibial glide | Flexion moves the tibia posteriorly on the femur → concave on convex → glide SAME direction → posterior |
Key teaching point: The convex-concave rule gives opposite glide directions depending on which surface moves. At the GH joint (convex humerus moves), glide is OPPOSITE to the restriction. At the tibiofemoral joint (concave tibia moves), glide is in the SAME direction as the restriction. Students commonly confuse this — always identify which surface is moving before determining glide direction.
General Contraindications (All Tibiofemoral Mobilizations)
- Absolute: Ligamentous instability (ACL/PCL/MCL/LCL compromise), acute fracture, active joint infection, malignancy, joint effusion with acute onset (rule out hemarthrosis), acute meniscal lock
- Relative (modify grade): Significant OA with bony end-feel (Grade I–II only; Grade III–IV into a bony barrier is contraindicated), osteoporosis, total knee replacement (follow surgeon protocol), Baker's cyst (avoid forced flexion)
- Grade-specific: If Grade I–II oscillations produce pain, do not progress to Grade III–IV
Anterior Tibial Glide
Purpose: Restores knee extension. The anterior tibial glide moves the concave tibial surface anteriorly on the convex femoral condyles, reproducing the arthrokinematic component of extension. Used when extension is limited by posterior capsular tightness (capsular end-feel on PROM into extension). Patient position:- Supine on the treatment table
- Knee positioned at approximately 25° flexion (resting position)
- A rolled towel or bolster under the distal femur (just proximal to the knee) elevates the thigh and allows the tibia to move freely
- The foot rests on the table or hangs slightly off the edge
- Stabilizing hand: Applied to the anterior distal femur, pressing it into the bolster. This prevents the femur from sliding posteriorly when force is applied to the tibia. The stabilizing force is directed posteriorly.
- Mobilizing hand: Both hands wrap around the proximal tibia, fingers interlaced on the posterior calf just below the joint line. The thenar eminences contact the posterolateral and posteromedial tibial condyle regions. The force is directed anteriorly.
- Apply a slow, sustained or oscillatory force directed anteriorly (toward the ceiling with the patient supine) to the proximal tibia
- Grade I–II (pain modulation): Small to large amplitude oscillations within the pain-free range. For acute post-surgical or post-injury stiffness where extension is painful.
- Grade III–IV (mobility): Large to small amplitude oscillations at the end of available extension range, into the posterior capsular resistance. You should feel the capsule's firm barrier and oscillate into it.
- Rhythm: slow, rhythmic oscillations at approximately 1–2 per second
- Duration: 30–60 seconds per set, 3–5 sets. Reassess extension PROM between sets.
- Capsular end-feel on PROM into extension
- Extension loss following the capsular pattern (flexion > extension, meaning both are restricted but flexion is proportionally more limited)
- Post-immobilization stiffness (cast, brace, prolonged bed rest)
- Post-surgical extension deficit (ACL reconstruction, meniscal repair — only when cleared by surgeon)
- Common error: Hands too far distal — creates torque, not translatory glide. Keep hands just below the joint line.
- Common error: Failing to stabilize the femur — the entire leg translates with no arthrokinematic motion.
- Reassessment: Re-test extension PROM. Improvement of 3-5° per session is reasonable.
- Integration: Perform after hamstring release (especially popliteal fossa tendons). If end-feel remains capsular after hamstring release, proceed with mobilization.
Posterior Tibial Glide
Purpose: Restores knee flexion. The posterior tibial glide moves the concave tibial surface posteriorly on the convex femoral condyles, reproducing the arthrokinematic component of flexion. Used when flexion is limited by anterior capsular tightness. Patient position:- Supine on the treatment table
- Knee positioned at approximately 25° flexion (resting position), progressing toward available end-range flexion as tolerance allows
- The foot rests flat on the table with the knee bent (hook-lying position is often comfortable)
- Alternatively: seated at the edge of the table with the knee hanging in flexion (gravity-assisted position)
- Stabilizing hand: Applied to the posterior distal femur or the table provides stabilization. In the supine hook-lying position, the femur is stabilized by body weight and table contact.
- Mobilizing hand: Heel of the hand or thenar eminence contacts the anterior proximal tibia (tibial tuberosity region). The force is directed posteriorly.
- Apply a slow, sustained or oscillatory force directed posteriorly to the proximal tibia
- Grade I–II: Small oscillations within pain-free range — for pain modulation after acute injury or surgery
- Grade III–IV: Oscillations at end-range flexion, into the anterior capsular resistance. The anterior capsule and quadriceps tendon will provide a firm barrier.
- Same rhythm, duration, and set parameters as the anterior tibial glide
- Capsular end-feel on PROM into flexion
- Flexion loss following the capsular pattern
- Post-surgical flexion deficit (total knee replacement — the most common indication, often targeted at achieving 90° flexion for functional activities like sitting and stair climbing, and 120° for activities like kneeling)
- Chronic flexion limitation from OA or post-immobilization adhesion
- Common error: Force directed downward into the tibia rather than posteriorly along the tibial surface.
- End-range positioning: Increase the starting flexion angle as ROM improves — oscillations must be at the current end-range.
- Reassessment: Functional milestones: 65° walking, 90° stair climbing, 105° rising from low chair, 120° kneeling.
- Integration: Perform after quadriceps release. If end-feel changes after quad release, the restriction was muscular (possibly AMI — see Clinical Notes). If it remains capsular, mobilization is indicated.
Patellar Mobilizations
Purpose: Restores patellofemoral mobility, which is essential for normal tibiofemoral mechanics. The patella must glide freely in the femoral trochlear groove for smooth knee flexion and extension. Patellar hypomobility contributes to anterior knee pain, difficulty with stair climbing, and post-surgical stiffness. Patient position:- Supine on the treatment table
- Knee in full extension or 20–30° flexion (a small bolster under the knee)
- The quadriceps must be fully relaxed — if the quads are contracting, the patella is compressed into the trochlear groove and cannot be mobilized
- Both thumbs or the web space of one hand contacts the lateral or medial border of the patella
- For superior/inferior glides, contact the superior or inferior pole
- Force directed medially (toward the opposite knee)
- Restores medial patellar mobility — often restricted in lateral patellar tracking or ITB tightness
- Particularly important for patients with patellofemoral syndrome where lateral tracking is present
- Force directed laterally (away from the opposite knee)
- Less commonly restricted; assess before treating
- Caution: do not aggressively mobilize laterally if lateral patellar tracking or subluxation is present — this worsens the problem
- Force directed superiorly (toward the hip)
- Restores quadriceps excursion — limited after distal quadriceps or patellar tendon surgery
- Important for restoring terminal knee extension
- Force directed inferiorly (toward the foot)
- Restores knee flexion — the patella must glide inferiorly in the trochlear groove as the knee flexes
- This is the most commonly restricted patellar direction after knee surgery or prolonged immobilization
- Grade I–II: Gentle oscillations for pain modulation; appropriate immediately post-surgery when cleared by surgeon
- Grade III–IV: Sustained pressure or oscillations at end-range for mobility
- Duration: 30–60 seconds per direction, 2–3 sets
- Decreased patellar mobility on accessory motion testing (always compare to the uninvolved side)
- Anterior knee pain with patellofemoral crepitus
- Post-surgical or post-immobilization knee stiffness (patella "stuck" in the groove)
- As an adjunct before tibiofemoral mobilization — restoring patellar mobility first often makes tibiofemoral techniques more effective
- Assessment first: Assess all four directions before mobilizing. Compare to uninvolved side. Mobilize only restricted directions.
- Common error: Quadriceps contracted — patella cannot move when quads pull it into the groove.
- Common error: Excessive force — the patella is a sesamoid bone; Grade I-II is usually sufficient.
- Effusion check: Significant effusion makes patellar testing unreliable (ballotable patella). Address effusion first.
Muscles Crossing This Joint
Extensors (Anterior)
- anatomy/muscles/quadriceps — rectus femoris, vastus lateralis, vastus medialis (VMO), vastus intermedius — the only extensors of the knee; rectus femoris is the only quad that crosses the hip (biarticular); VMO provides medial patellar tracking stability
Flexors (Posterior)
- anatomy/muscles/hamstrings — biceps femoris (lateral), semimembranosus and semitendinosus (medial) — primary knee flexors; all biarticular except the short head of biceps femoris
- anatomy/muscles/gastrocnemius — crosses the knee posteriorly; weak knee flexor; its tightness can limit full knee extension
- anatomy/muscles/popliteus — the "key to unlocking the knee" — initiates IR of the tibia to disengage the screw-home mechanism at the beginning of flexion from full extension
Medial Rotators (in flexion)
- anatomy/muscles/popliteus (primary tibial IR), anatomy/muscles/semitendinosus, anatomy/muscles/semimembranosus, anatomy/muscles/sartorius, anatomy/muscles/gracilis
Lateral Rotators (in flexion)
Pes Anserine Group
- anatomy/muscles/sartorius, anatomy/muscles/gracilis, anatomy/muscles/semitendinosus — these three muscles insert together on the anteromedial proximal tibia (the pes anserine). Their common tendinous insertion is a frequent site of tendinopathy and bursitis, especially in runners and individuals with pes planus or valgus knee alignment.
Conditions Affecting This Joint
- conditions/osteoarthritis — the most common tibiofemoral condition; medial compartment most frequently affected; follows the capsular pattern (flexion > extension); crepitus, morning stiffness, progressive ROM loss
- conditions/acl-injury — non-contact deceleration injury, giving-way episodes, hemarthrosis within 2 hours; positive Lachman's and anterior drawer
- conditions/mcl-injury — valgus mechanism, medial joint line tenderness, positive valgus stress test; most commonly injured knee ligament
- conditions/meniscal-injuries — rotational mechanism on loaded flexed knee, joint line tenderness, locking/catching, positive McMurray's and Apley's compression
- conditions/patellofemoral-syndrome — anterior knee pain worsened by stairs, squatting, prolonged sitting; patellar tracking dysfunction, Q-angle significance
- conditions/itb-syndrome — lateral knee pain at 30° flexion (Gerdy's tubercle / lateral femoral epicondyle); common in runners; positive Ober's test and Noble compression test
Clinical Notes
- Screw-home mechanism (terminal rotation). During the final 15-30° of extension, the tibia externally rotates ~5-10° due to asymmetric femoral condyles (the medial condyle is longer). This locks the knee in full extension. The popliteus unlocks it by internally rotating the tibia. Disruption (ACL deficiency, meniscal damage, OA) produces giving-way during terminal extension.
- Q-angle significance. Measured from ASIS through patella center to tibial tuberosity. Normal: males 10-15°, females 15-20°. Increased Q-angle produces greater lateral patellar pull, predisposing to patellofemoral pain and ACL injury. Females' wider pelvis explains their higher incidence.
- Arthrogenic quadriceps inhibition (AMI). Following knee injury, surgery, or effusion, the nervous system reflexively suppresses quadriceps activation. This is neurological, not simple disuse weakness. Results in VMO atrophy, extension lag, and altered gait. Stretching and mobilization will not resolve AMI — neuromuscular electrical stimulation and specific VMO activation exercises are needed.
- Assessment sequence for knee restriction. (1) Capsular pattern (flexion > extension)? Suspect OA or capsulitis. (2) Mechanical block (springy end-feel)? Suspect meniscal lock. (3) Ligamentous laxity (soft end-feel on stress testing)? Identify which ligament. (4) Extension loss muscular or capsular? Test with hamstring release.
- Meniscal considerations for mobilization. If a meniscal tear is suspected, avoid rotational components and use Grade I-II only until status is confirmed. Mobilizing into a meniscal block is contraindicated.
Key Takeaways
- Convex-concave rule at the knee: concave tibia on convex femur means glide is in the same direction as the restriction — opposite to the GH joint rule. This is the most commonly confused concept.
- Capsular pattern flexion > extension distinguishes OA/capsulitis from ligamentous or meniscal problems (non-capsular patterns).
- AMI is neurological, not simple weakness — stretching and mobilization will not resolve it. Identify it and use neuromuscular strategies.
- Medial meniscus is torn 3:1 over lateral because its MCL attachment limits mobility — meniscal integrity must be assessed before mobilization.
- Patellar mobility is a prerequisite for normal tibiofemoral mechanics — always assess and restore patellar glide alongside tibiofemoral mobilization.