Adult Chondral Injury of the Knee

The picture varies with mechanism, defect size, and location. Patients describe an acute injury or insidious activity-related pain localised to the affected compartment, intermittent effusion (especially after activity), and mechanical symptoms suggesting a loose body or unstable flap. Recurrent effusion with catching points toward chondral pathology rather than meniscal; a true mechanical block to extension points back toward meniscal tear or loose body. Pain on stairs (especially descending) and squatting suggests patellofemoral involvement. Functional limitation is often disproportionate to imaging severity.

Examination findings include an effusion (sweep test, patellar tap), joint-line tenderness over the affected compartment, pain on deep flexion or full extension depending on lesion location, and patellofemoral signs (patellar grind, lateral tilt) for trochlear or patellar lesions. Always assess associated pathology: ACL (Lachman, anterior drawer, pivot shift), meniscus (McMurray, Thessaly), patellar instability (apprehension, J-sign), and quadriceps wasting in chronic cases. Range of motion is usually preserved except in large defects with mechanical locking.

Differentials: meniscal tear (true locking, joint-line tenderness, positive McMurray/Thessaly), ACL rupture (acute haemarthrosis, positive Lachman), recurrent patellar dislocation, osteochondritis dissecans (lateral aspect of medial femoral condyle in young adults), early OA, patellofemoral pain syndrome, inflammatory arthropathy, and septic arthritis.

Plain weight-bearing radiographs (AP, lateral, skyline or Merchant, Rosenberg or PA flexed view) are first-line. They are useful for joint-space narrowing, osteophytes, alignment, loose bodies, and osteochondral fragments. A purely chondral defect is radiographically silent.

MRI is the investigation of choice for chondral and osteochondral injury. Cartilage-sensitive proton density fat-suppressed sequences are the main NHS workhorse. T2 mapping and T1rho are increasingly available compositional techniques; dGEMRIC is largely a research tool because it requires IV gadolinium and a 90-minute delay. MR or CT arthrography is reserved for selected surgical planning when MRI is equivocal.

Diagnostic arthroscopy remains the gold standard for direct visualisation, lesion grading, and probing for stability, but is rarely first-line because MRI is now sufficient for most surgical decisions. Diagnostic or therapeutic arthroscopy may be appropriate for a focal unstable chondral flap, loose body, displaced osteochondral fragment, or unexplained true locking. NICE NG226 advises against arthroscopic lavage or debridement for osteoarthritis - this is a different scenario and must not be confused with focal mechanical pathology.

Aspiration is useful when effusion is significant. A lipohaemarthrosis (fat globules on aspirate or a fat-fluid level on imaging) strongly suggests an intra-articular fracture, including osteochondral fracture. The lesion-assessment checklist for surgical planning covers size, depth, location, containment, subchondral bone, alignment, meniscus, ligaments, patellar tracking, and Kellgren-Lawrence OA grade (ACI requires KL 0-2 under NICE TA477).

Treatment depends on lesion size, depth, location, age, activity demand, alignment, ligament status, and meniscal status. Treatment is matched to the lesion and the patient.

First-line conservative management (small or asymptomatic Outerbridge I-II lesions, or any patient unfit for surgery): activity modification to reduce impact loading, quadriceps and hip strengthening under physiotherapy guidance with neuromuscular control work, weight optimisation per NICE NG226 OA principles, simple analgesia and short-course NSAIDs per BNF (GI and cardiovascular caution), and bracing or an unloader for symptomatic medial or lateral compartment lesions with malalignment. Address contributors: alignment, instability (ACL), meniscal status, RED-S in young athletes, and vitamin D. Injections may help symptom modulation but do not restore cartilage; NICE NG226 advises against intra-articular hyaluronan for OA, and intra-articular corticosteroid offers only short-term relief.

Acute osteochondral fracture with a displaced fragment is an urgent orthopaedic referral. Fixation of the fragment is preferred over excision when the fragment is large enough and the bone is viable, particularly in younger patients (headless compression screws or bioabsorbable implants).

Surgical options for symptomatic high-grade defects (Outerbridge IV / ICRS 3-4) follow a size-based pathway. Size thresholds are exam heuristics, not rigid law:

• •Microfracture (marrow stimulation): small contained lesions under 2 cm squared. Subchondral perforations recruit marrow stem cells producing fibrocartilage. First-line use is declining for larger lesions and high-demand athletes due to durability concerns.
• •AMIC (scaffold-augmented marrow stimulation): around 1.5-3 cm squared. Single-stage; a collagen membrane over a microfractured bed traps stem cells and improves repair quality.
• •OATS / mosaicplasty (osteochondral autograft): small to medium lesions, 1-4 cm squared. Cylindrical osteochondral plugs from non-weight-bearing donor sites (lateral or medial margins of the intercondylar notch, superolateral trochlea). Donor-site morbidity is the main limitation.
• •ACI / MACI (autologous chondrocyte implantation): typically above 2 cm squared. Two-stage; autologous chondrocytes are harvested, expanded ex vivo, and implanted into the defect beneath a membrane or on a scaffold.
• •Osteochondral allograft: large defects, deep osteochondral lesions, or salvage.

NICE TA477 criteria (know cold): no previous cartilage repair surgery; minimal OA (Kellgren-Lawrence 0-2); defect over 2 cm squared; procedure at a tertiary referral centre. Failed previous marrow stimulation is NOT a TA477 criterion - do not state this.

Concomitant pathology must be addressed in the same operative episode: malalignment (high tibial osteotomy for varus, distal femoral osteotomy for valgus), patellar maltracking or instability (MPFL reconstruction, tibial tuberosity transfer, trochleoplasty as indicated), ligament instability (ACL reconstruction), and meniscal deficiency (repair or allograft transplantation in selected cases). A cartilage repair done in a malaligned, unstable, or meniscal-deficient knee will fail - the single most important management principle in this topic. VTE risk assessment per NICE NG89 for any post-operative immobilisation.

Rehabilitation is lesion-, site-, and procedure-specific, with exact protocols varying between UK units.

Conservative pathway: quadriceps and hip strengthening, neuromuscular control, and gait re-education, with closed-chain over open-chain initially in the painful range. Progressive return to impact is based on symptoms, effusion, and strength. Address contributors (weight, alignment-related load, sport technique) throughout.

Post-operative microfracture / scaffold-augmented microfracture / OATS: 0-6 weeks protected weight-bearing per surgeon (often touch or partial WB for femoral condyle lesions; WB as tolerated for patellofemoral lesions in extension), early CPM if available, and progressive ROM. 6-12 weeks progressive weight-bearing, closed-chain strengthening, balance, and proprioception. 3-6 months progressive impact and sport-specific drills. Return to running is typically 4-6 months, with return to pivoting or contact sport at 9-12 months for most patients.

Post-operative ACI / MACI: cell maturation drives a slower protocol; weight-bearing and sport timelines are extended versus microfracture. Return to high-impact sport is often 12-18 months. Return-to-sport decisions are imaging- and function-based, not time-based alone.

Outcomes vary with lesion size, location (patellofemoral lesions tend to do worse than condylar), age, BMI, alignment, and adherence.

• •NICE TA477 - Autologous chondrocyte implantation (ACI/MACI) is recommended for symptomatic articular cartilage defects of the knee where no previous cartilage repair has been performed, OA is minimal (KL 0-2), the defect is over 2 cm squared, and surgery is at a tertiary referral centre.
• •NICE NG226 (2022) - Osteoarthritis: arthroscopic lavage and debridement are NOT recommended for OA; intra-articular hyaluronan is NOT recommended; intra-articular corticosteroid offers only short-term relief (around 2-10 weeks).
• •NICE NG89 - VTE risk assessment is mandatory in any patient with lower-limb immobilisation following cartilage surgery.
• •BASK / ESSKA cartilage committee consensus - size-based algorithm for cartilage repair, with mandatory correction of concomitant malalignment, instability, and meniscal deficiency.
• •Knutsen et al. (2016, JBJS) - Long-term follow-up showing comparable outcomes between microfracture and first-generation ACI; treatment failure in both groups is driven by uncorrected concomitant pathology.

• •Cartilage has no blood supply, no nerves, and limited intrinsic healing - the principle that drives every management decision. A defect that does not breach the subchondral plate effectively does not heal.
• •Outerbridge is arthroscopic with four grades only (IV is full-thickness, exposed bone); ICRS maps broadly. Plain X-rays are normal in pure chondral injury - do not be falsely reassured.
• •A lipohaemarthrosis (fat-fluid level on imaging or fat globules on aspirate) strongly suggests intra-articular fracture, often osteochondral. Classic patellar dislocation chondral pattern = medial patellar facet plus lateral femoral condyle.
• •Surgical options by lesion size: microfracture (under 2 cm squared), AMIC (1.5-3 cm squared), OATS (1-4 cm squared), ACI/MACI (above 2 cm squared, per NICE TA477), allograft (large or salvage).
• •NICE TA477 four criteria cold: no previous cartilage repair surgery, minimal OA (KL 0-2), defect over 2 cm squared, tertiary centre. Failed previous microfracture is NOT a TA477 criterion.
• •A cartilage repair done in a malaligned, unstable, or meniscal-deficient knee will fail - address all concomitant pathology in the same operative episode.