During knee replacement surgery, muscles are generally not cut through and reattached. Instead, surgeons work around the muscles by making incisions through tendons, the joint capsule, and a tough fibrous layer called the retinaculum. These soft tissues are then sutured closed in layers at the end of surgery, and your body heals them back together over the following weeks and months. The specific tissues that get cut, and how they’re repaired, depend on which surgical approach your surgeon uses.
What Actually Gets Cut Open
The front of your knee is dominated by the quadriceps muscle group, which connects to your kneecap through the quadriceps tendon above and the patellar tendon below. To access the joint, surgeons need to get past this extensor mechanism without destroying it. The most common technique, called the medial parapatellar approach, makes an incision along the inner edge of the kneecap through the retinaculum (a sheet of connective tissue that holds the kneecap in place) and extends it upward into the quadriceps tendon. The surgeon leaves a small cuff of tendon tissue, about 3 to 4 millimeters wide, still attached to the inner thigh muscle. This cuff is critical because it gives the surgeon something to stitch back to during closure.
The incision continues down along the inner border of the patellar tendon and onto the shinbone about 3 to 4 centimeters. Once open, the kneecap is flipped to the side, exposing the entire joint for the surgeon to work on the bone surfaces.
Approaches That Spare More Tissue
Two alternative approaches reduce how much tissue gets cut. In the midvastus approach, the surgeon splits the inner thigh muscle along the direction of its fibers using blunt dissection rather than a sharp cut. Because muscle fibers are separated rather than severed, they fall back together more naturally afterward. The quadriceps tendon itself is not cut at all.
The subvastus approach goes even further. The surgeon works underneath the inner thigh muscle by detaching its lower edge from the tissue it sits on, then entering the joint below the muscle belly. This leaves the muscle essentially intact. Both of these techniques are associated with less postoperative pain and quicker early recovery, though they can be technically harder to perform, especially in patients with larger or stiffer knees.
A quadriceps-sparing variation limits the tendon incision to just 2 centimeters above the kneecap, compared to the 6 to 8 centimeters typical in conventional surgery. The kneecap is shifted to the side rather than fully flipped over, which reduces stress on the surrounding soft tissues.
How the Tissues Are Closed
Closure is done in layers, starting deep and working outward. The joint capsule is sutured first to seal the joint space. Then the retinaculum on the inner side of the kneecap is repaired, restoring the structure that keeps the kneecap tracking properly in its groove. This layer matters because it acts as the primary restraint preventing the kneecap from shifting outward.
Where the quadriceps tendon was split, the surgeon stitches the two edges back together. The small cuff of tendon left on the inner thigh muscle during the initial incision now serves as an anchor point. Surgeons typically use strong, non-absorbable sutures in a whipstitch pattern, which weaves the thread back and forth through the tissue for a secure grip. In some cases, particularly when extra strength is needed, a locking stitch pattern called a Krakow technique is used, where the suture zigzags through the tendon in a way that tightens under load rather than pulling through.
The repair is done with the knee bent to about 30 degrees so the surgeon can confirm that the kneecap sits centered in its groove and that the tension on the soft tissues is balanced. If the repair is too tight on the inner side, the kneecap won’t glide smoothly. If it’s too loose, tracking problems can develop later.
How Your Body Heals the Repair
Sutures hold everything in position, but your body does the actual reattaching. The healing process follows a predictable sequence: inflammation kicks in first, bringing blood flow and immune cells to the repair site. New blood vessels form in the area over the following days and weeks. Cells then migrate into the repair zone, laying down new collagen to bridge the gap. Finally, this new tissue remodels over months, gradually becoming stronger and more organized.
Imaging studies of surgical tendon repairs in younger patients show measurable new tissue forming at the repair site within three months, with continued thickening through six months. Older patients and those with other health conditions heal more slowly. Full maturation of the repaired tissue takes considerably longer, which is why surgeons impose activity restrictions well beyond the point where you start feeling better.
Why Your Quadriceps Feel So Weak Afterward
Even though the muscles themselves are mostly preserved during surgery, your quadriceps will feel dramatically weaker afterward. At two weeks post-surgery, quadriceps strength drops by roughly 37.5% compared to pre-operative levels. Even at three months, strength is still significantly below where it was before surgery.
Part of this is expected surgical recovery, but a major contributor is something called arthrogenic muscle inhibition. Your nervous system essentially shuts down full activation of the quadriceps in response to joint swelling, pain, and tissue damage. Your brain is trying to protect the knee, but the result is that you physically cannot contract the muscle fully, no matter how hard you try. This is not a willpower issue.
Pain is the strongest driver of this inhibition. Patients with high pain scores in the first two weeks are over 13 times more likely to develop persistent quadriceps shutdown. Habits that seem comforting, like placing a pillow under your knee or keeping the hospital bed bent so your knee stays flexed, nearly six times the risk. These positions let the hamstrings tighten, which further suppresses the quadriceps. Keeping your knee straight when resting and staying on top of pain management in those early days makes a meaningful difference in how quickly your quadriceps come back online.
What Can Go Wrong With the Repair
Failure of the extensor mechanism, meaning the quadriceps tendon, patellar tendon, or their attachment to the kneecap breaks down, occurs in 0.1% to 2.5% of knee replacements. Revision surgeries carry a higher risk than first-time procedures because the tissues have already been cut and repaired once, and scar tissue doesn’t hold sutures as reliably as healthy tissue.
Signs of a problem include a sudden inability to straighten your knee or lift your leg, a visible gap above or below the kneecap, or the kneecap sitting noticeably higher or lower than normal. This is a serious complication that typically requires another surgery to repair. The risk is highest in the first several weeks while the tissue is still healing, which is why physical therapy protocols start with gentle range-of-motion exercises and progress gradually to resistance work over months rather than weeks.
The Recovery Arc
Your surgeon’s rehabilitation protocol is designed around the biology of tissue healing and the challenge of overcoming muscle inhibition. In the first two weeks, the focus is on pain control, reducing swelling, and gently bending and straightening the knee to prevent stiffness. Quadriceps activation exercises, like tightening the thigh muscle while lying flat, begin almost immediately even though you won’t be able to generate much force.
By six weeks, the soft tissue repair has gained enough strength that more active exercises are introduced. Most people are walking without significant assistive devices by this point, though the quadriceps remain substantially weaker than before surgery. The three-to-six-month window is where the most meaningful strength gains happen, as the repaired tissues mature and the nervous system gradually releases its protective inhibition. Full recovery of quadriceps strength, to the extent it returns, often takes six months to a year. Factors that slow this timeline include higher body weight, older age, and female sex, all of which are associated with slower strength recovery in large analyses of knee replacement outcomes.