The meniscus, and what it does for cartilage

The menisci are two crescents of fibrocartilage sitting between the femur and the tibia, anchored at both horns and around the rim. Their collagen runs circumferentially, which is the whole trick: an axial load pushing down on a wedge-shaped ring gets converted into tension around the ring rather than pressure straight through it. That is what spreads a load across the cartilage instead of concentrating it. The medial meniscus is tethered at more points than the lateral one, and that is the usual explanation for why it is the one that tears.

What resection costs, measured

The mechanical bill comes due immediately, and it has been measured in loaded human cadaver knees. Partial meniscectomy dropped contact area about 10% and raised peak local contact stress about 65%. Total meniscectomy dropped contact area about 75% and raised peak local contact stress about 235%. In the same experiment, repairing a peripheral tear left contact area and stress where they had been, whichever suture technique was used, while resecting that same segment raised peak local contact stress about 110%.

A more recent gait simulator refines the picture in a way that matters clinically. Peak tibial contact pressure in an intact knee was 6 MPa at 14% of the gait cycle, and neither the size nor the position of that peak changed for radial tears involving up to 60% of the meniscal rim width. A tear taking 90% of the width shifted the peak posterocentrally and added 1.3 MPa; partial meniscectomy of the same tear then added a further 1.4 MPa on top. A torn meniscus is still carrying load. It is not mechanically equivalent to no meniscus, which is the assumption that makes resection look free.

Which tears can heal

Blood supply decides it. A perimeniscal capillary plexus, arising in the capsular and synovial tissues of the joint, reaches only the peripheral 10-25% of the meniscus. Everything inside that is avascular. The posterolateral segment of the lateral meniscus, beside the popliteal tendon, has neither penetrating vessels nor even a synovial fringe, while both horn attachments sit under vascular synovial tissue.

This is the anatomy behind the red-red, red-white and white-white zones a surgeon uses to decide whether a tear is worth stitching. Worth being precise about what the mapping study did and did not do: it traced where blood arrives. It did not measure healing, and no part of the zone system is a healing rate.

What a tear on a scan means

Nine hundred and ninety-one people in Framingham, aged 50 to 90, were scanned without being selected for knee trouble. A meniscal tear or meniscal destruction turned up in 19% of women aged 50 to 59 and 56% of men aged 70 to 90. Among people with radiographic osteoarthritis, 63% of those with knee pain on most days had a tear — and so did 60% of those without. Without radiographic osteoarthritis, the figures were 32% and 23%.

Sixty-one percent of everyone found to have a tear had reported no pain, aching or stiffness in the previous month.

That is the base rate any MRI report has to be read against. In a knee past fifty, finding a meniscal tear is close to finding a grey hair: real, visible, and not by itself the reason anything hurts.

What the long cohorts show

Sixteen years after an isolated limited meniscectomy, in 155 patients with intact cruciate ligaments, radiographic tibiofemoral osteoarthritis was present in 43% of index knees and half the patients had a symptomatic index knee. Set against matched controls, the relative risk of combined radiographic and symptomatic osteoarthritis was RR 7.0 (95% CI 2.1-23.5) where the tear had been degenerative, and 2.7 (95% CI 0.9-7.7) where it had been traumatic. The authors' own reading of that split is worth carrying: a degenerative tear may be the first sign of an osteoarthritic process rather than only its cause.

A separate cohort of 221 people, meniscectomised 15 to 22 years earlier and then radiographed twice over 4 to 10 years, watched tibiofemoral osteoarthritis in the index knee climb from 48% to 68%, and patellofemoral from 14% to 23%.

How much was taken out tracks the outcome. Fourteen years on, in 36 matched men, radiographic changes appeared in 6 of 18 knees (33%) after partial meniscectomy and 13 of 18 (72%) after total meniscectomy, against 4 of 36 (11%) in uninjured controls. A systematic review of 4 randomised trials and 25 cohorts found the same ordering — total resection, removal of the peripheral rim, lateral rather than medial resection, a degenerative tear, existing chondral damage and higher body mass index all predicted a worse result.

Where the imaging clock runs fastest

In the Osteoarthritis Initiative, every one of the 31 knees that had a partial meniscectomy went on to radiographic osteoarthritis, as did 58.9% of knees with prevalent meniscal damage (OR 2.51). Among knees that developed it, a partial meniscectomy in the preceding year carried an OR of 4.51 for worsening cartilage damage.

A matched analysis in the same cohort measured the shape of the loss, and the shape is the interesting part. Joint space width fell 0.083 mm per month over the first twelve months after meniscectomy — roughly 27 times the rate in knees with an untreated tear, and 5 times the rate in knees with no tear. After that the three rates converged. At 72 months, adjusted joint space width was 2.57 mm after meniscectomy, 3.73 mm with an untreated tear and 3.54 mm with no tear. The damage is front-loaded, and then the knee carries the gap forward.

The one randomised structural result

Everything above is observational, and observational evidence here has a specific weakness: the knee that gets operated on is the knee that hurt more, and a knee that hurts more may have been heading somewhere worse anyway.

One trial escapes that, because it randomised. In the placebo-surgery controlled FIDELITY trial, 72% of knees having an arthroscopic partial meniscectomy and 60% of knees having a sham procedure had progressed at least one Kellgren-Lawrence grade at five years — an adjusted absolute risk difference of 13%, with a confidence interval running from -2% to 28%. The MRI arm of that same follow-up then located the signal: odds ratios in the operated arm were 2.86 for osteophytes, 1.31 for cartilage damage and 1.43 for bone marrow lesions, and only the osteophyte interval excluded 1.

So the randomised evidence supports the direction and is not precise about the size, and the most cartilage-specific measurement in it has not yet been replicated.

What would sharpen this

The surgeon's actual decision is how much meniscus to take, and nobody has randomised it. The cadaver work brackets the answer at nothing and everything, the gait simulator brackets tear size, and the long cohorts consistently rank resection volume as the strongest predictor they have — which is a lot of converging evidence about a variable no trial has ever allocated. A dose-response study of resection volume would tell a reader facing this operation the one number nobody can currently give them.