Rehab after cartilage repair

A MACI graft or microfracture clot is mechanically fragile in the first weeks and matures over 6–18 months. Rehab's job is a genuine tightrope: protect the repair from loads it cannot yet take, while supplying the cyclic motion and compression cartilage needs to mature — because total immobilization is its own harm, thinning even healthy cartilage within weeks. What follows is what's actually tested versus what's tradition.

The one real weight-bearing trial

Sixty-three patients after MACI to femoral condyle defects were randomized to an accelerated protocol reaching full weight-bearing at 8 weeks or a traditional one reaching it at 11 weeks. At five years, the accelerated group had less knee pain and no worse graft outcomes, with 94–95% satisfaction in both arms. Faster — but still graded — loading was safe. It's worth noting what rehab is protecting: in the SUMMIT RCT (n=144), MACI beat microfracture on pain and function at two years and held that lead at five, with both arms following structured protected-loading protocols.

CPM: ubiquitous, unproven in humans

Continuous passive motion machines appear in nearly every cartilage-repair protocol. The human evidence base is four Level III studies from which no definitive conclusion could be drawn. The actual evidentiary basis is a rabbit-model literature — Salter's motion-without-compression rationale — where CPM improved cartilage nutrition and repair quality versus immobilization. That is real basic science, and it is animal data; human benefit is unproven despite the protocol ubiquity.

The protocol skeleton

From trial arms and reviews — not validated element-by-element: early range of motion and CPM from weeks 0–6; protected weight-bearing progressing to full over about 8 weeks for femoral condyle MACI (the accelerated arm); strengthening at low joint load early, progressing isometrics → BFR → progressive resistance; and impact and sport reintroduced from roughly 9–18 months by criteria — strength symmetry and effusion-free loading tolerance — rather than the calendar alone. Patellofemoral and larger lesions run slower than this; the trial data are condyle-centric.

The strength model comes from adjacent surgery: in ACL-reconstruction patients, low-load BFR matched heavy-load training for strength while beating it on function, range of motion, pain, and effusion. No RCT tests BFR or isometrics in cartilage-repair patients specifically — that piece is extrapolated.

What the outcome literature says to expect

Return-to-sport reviews across 1,117 athletes report rates of 75% (microfracture) to 84–89% (ACI, osteochondral grafting), typically at 9–18 months. For microfracture, a 28-study review found reliable improvement in the first 24 months with conflicting durability after — which complicates any tidy definition of "successful rehab" and sets honest expectations for the marrow-stimulation pathway.

Limitations and safety

The accelerated weight-bearing RCT is a single-center trial in femoral condyle MACI; it does not license early loading for every lesion type. Return-to-sport timelines are descriptive, pooled from heterogeneous studies — not tested targets. Graded accelerated loading showed no graft harm at five years, but persistent effusion, rising pain, or mechanical symptoms during progression warrant surgical review rather than pushing through (clinical convention, not trial-derived), and unprotected impact loading before graft maturation is the line no protocol crosses.

Open questions

Element-level RCTs — CPM yes or no, weight-bearing speed for patellofemoral lesions — validated return-to-sport criteria, and BFR timing relative to weight-bearing restrictions.