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 weight-bearing trial after MACI

Seventy patients after MACI were randomised to an accelerated protocol reaching full weight-bearing at 8 weeks or a traditional one reaching it at 11 weeks. Both arms improved on every clinical measure to 24 months. The accelerated group reported significantly less severe pain, walked further in six minutes, and had greater maximal active knee extension. Nothing else separated.

The result that matters most is the one that did not happen: no graft delamination up to 24 months resulted from either three-month rehabilitation programme. Three weeks less on crutches cost nothing measurable, which is the whole question the trial was built to answer.

Its limits are real. Seventy patients at one centre, nobody blinded to which protocol they were following, and two years is short for a cartilage graft. It shows that this particular acceleration is safe; it does not show that faster is generally better. 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.

Continuous passive motion

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. What sits underneath is a rabbit literature — nineteen studies, reviewed together — and it is more specific than the rationale usually attributed to it.

Against an immobilized knee, CPM did what Salter's work claimed: flexion contracture developed in 0–13% of CPM knees against 40–100% of immobilized ones, and hyaline cartilage was the predominant repair tissue in 70–100% of CPM defects against 8–79% of immobilized ones. Against a rabbit simply left to move around its cage, the functional advantage disappeared — no difference in contracture between one and ten weeks, and none in any group at a year.

The mechanism is not the one usually named either. That same review found CPM did not increase uptake of low-molecular-weight nutrients into cartilage. What it did was clear the joint: sinusoidal pressure changes that pump blood and solutes out, alongside suppressed interleukin-1, interleukin-6 and TNF-α. This is the sharpest available illustration of a distinction the loading-science entry turns on — cyclic compression is what feeds cartilage, and passive motion without it drains and calms the joint instead.

That is real basic science, and it is animal data; only one of the nineteen studies followed beyond twelve weeks. 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 the expectation 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.