Inner knee pain after a knee replacement is one of the most common post-surgical complaints, and it has a long list of possible causes ranging from a slightly oversized implant component to a tiny nerve injury sustained during the operation itself. The medial side of the knee (the side closest to your other leg) is especially vulnerable because the surgical incision typically runs right over it, and the anatomy there includes ligaments, tendons, a fluid-filled sac called a bursa, and delicate nerve branches that can all be disrupted. Pinpointing the source matters because some causes resolve on their own, while others need targeted treatment or even revision surgery.
The Tibial Component Might Be Slightly Too Wide
One of the more straightforward mechanical explanations is medial overhang of the tibial component, the metal tray that sits on top of your shinbone. If the edge of that tray extends even a few millimeters beyond the bone on the inner side, it can press against the soft tissues around the knee. A study looking at this relationship found that medial overhang was significantly associated with worse pain scores a year after surgery.1PubMed. Medial Overhang of the Tibial Component Is Associated With Higher Risk of Inferior Knee Injury and Osteoarthritis Outcome Score Pain After Knee Replacement The problem has also been documented in partial (unicompartmental) knee replacements, where excessive protrusion of the tibial implant irritates medial soft tissue and worsens the patient’s functional outcome.2The Knee. Effect of medial tibial component overhang on postoperative short-term revision rate and clinical outcomes in Oxford mobile-bearing unicompartmental knee arthroplasty
This is not always easy to spot. Standard X-rays can show whether the component overhangs the bone, but a millimeter or two of overhang might look trivial on film while still causing real discomfort in daily life. If your surgeon suspects this is the issue, a CT scan usually gives a clearer picture. The fix, unfortunately, can require a revision operation to swap in a better-fitting component, though some patients find that the irritation settles over time as the surrounding tissue adapts.
Damage to a Small Nerve You Have Probably Never Heard Of
Running along the inner side of your knee is a small nerve called the infrapatellar branch of the saphenous nerve. It crosses right through the zone where the surgeon makes the standard midline incision and is frequently cut or stretched during surgery. When the severed end of a nerve tries to heal, it sometimes forms a small, painful lump called a neuroma. This is more common than you might expect. A case report documented a 70-year-old woman who had four years of medial knee pain and stiffness after her replacement; physical exam showed tenderness on the inner knee with a positive Tinel’s sign (a tapping test that triggers tingling), and the diagnosis of a neuroma was confirmed when tissue was examined after surgical removal.3PubMed Central. Neuroma of the Infrapatellar branch of the saphenous nerve following Total knee Arthroplasty: a case report
Removing the neuroma can provide immediate pain relief and even improve range of motion in the knee.4PubMed Central. Surgical treatment outcome of painful traumatic neuroma of the infrapatellar branch of the saphenous nerve during total knee arthroplasty The tricky part is thinking to look for it in the first place. Many surgeons focus on the implant itself when evaluating post-replacement pain, and nerve problems can fly under the radar for months or years. If your pain is localized to the inner side of the knee, gets worse with light touch or clothing rubbing against it, and came on shortly after surgery, a neuroma is worth investigating.
Pes Anserine Bursitis
On the inner side of your knee, just below the joint line, three tendons from your hamstring muscles attach to the shinbone in a fan-shaped arrangement. Beneath those tendons sits a small fluid-filled sac, the pes anserine bursa. When it becomes inflamed, the result is a localized aching or burning pain on the inside of the knee that typically gets worse when you climb stairs or get up from a chair.
This can happen independently of knee replacement, but surgery can provoke it. Altered walking mechanics after your operation, swelling in the surrounding tissues, or direct surgical irritation of the area can all trigger the inflammation. Though described as rare immediately following total knee replacement, it has been documented and responds well to a steroid injection under ultrasound guidance.5PubMed Central. Painful Pes Anserine Bursitis Following Total Knee Replacement Surgery: Two cases The location of the tenderness is the main clue: it sits a couple of inches below the inner knee joint line, distinctly lower than where implant-related pain or neuroma pain tends to show up.
Medial Collateral Ligament Strain or Instability
The medial collateral ligament (MCL) runs along the inner edge of the knee, connecting your thighbone to your shinbone and keeping the joint stable against sideways forces. During knee replacement surgery, surgeons need to work around this ligament carefully. In rare cases, it can be stretched, partially torn, or weakened during the procedure. The result is a feeling of looseness or giving-way on the inner side of the knee, sometimes combined with pain.6PubMed Central. Medial collateral ligament reconstruction during TKA: a new approach and surgical technique
The surgical team balances the ligaments during the operation, releasing tighter structures so the knee moves evenly. If that balance is slightly off, or if the MCL was inadvertently injured, you may notice the instability most when walking on uneven ground or twisting. Treatment options range from bracing and physical therapy for mild looseness to surgical reconstruction using a tendon graft for more severe cases. A thicker polyethylene insert (the plastic spacer between the metal components) can also compensate for some degree of laxity without a full revision.
Scar Tissue and Soft Tissue Impingement
After any joint surgery, the body lays down scar tissue as part of healing. In some people, that scar tissue becomes excessive, forming thick bands of fibrous material that can get pinched between the moving parts of the implant. This is sometimes called arthrofibrosis when it leads to stiffness, or soft tissue impingement when it causes pain with specific movements. Arthroscopic inspection of painful knee replacements has revealed hypertrophic fibrous tissue and reactive synovial changes in the medial gutter, the groove along the inner wall of the joint.7Arthroscopy and Orthopedic Sports Medicine. Arthroscopic treatment of soft tissue impingement caused by extruded and a detached cement following total knee replacement arthroplasty
This kind of impingement tends to cause a catching or pinching sensation at certain angles, often during bending or straightening. Loose fragments of bone cement left behind from surgery can make the problem worse by acting as an irritant. When scar tissue is the main culprit, arthroscopic cleanup (removing the excess tissue through small incisions) can offer significant relief. The challenge is distinguishing it from other internal causes, which usually requires imaging plus a careful physical exam looking at exactly when in the bending arc the pain kicks in.
Low-Grade Infection
This is the cause nobody wants to hear about, but it is too important to skip. A low-grade infection around a knee replacement can simmer for months, causing vague, persistent inner knee pain along with mild swelling and stiffness that never quite improves the way it should. Unlike an acute infection, which tends to show up within days of surgery with obvious redness and fever, a low-grade infection may involve slow-growing bacteria that fly under the radar of routine blood tests.
Treating these infections is difficult. Surgical cleaning of the joint (irrigation and debridement) has inconsistent results, with failure rates that vary widely depending on the organism involved. In one review, the overall failure rate of debridement was around 45 percent, and outcomes were particularly poor when resistant bacteria were present or symptoms had been going on for three weeks or more.8PubMed Central. Low-grade periprosthetic knee infection: diagnosis and management In many cases, the most reliable treatment is a two-stage revision: the infected implant is removed, antibiotics are given over several weeks, and a new implant is placed once the infection has cleared. If your pain is accompanied by any persistent warmth, subtle swelling, or a wound that keeps draining, your surgeon should rule out infection before pursuing other diagnoses.
Patellar Tracking Problems
Your kneecap (patella) glides along a groove in the front of the implant every time you bend and straighten your knee. If the femoral or tibial component is rotated slightly inward, the kneecap can be pushed off its normal track. Internal rotation of these components is consistently associated with increased sideways tilt and shift of the kneecap.9PubMed Central. Patellar Instability Following Total Knee Arthroplasty: A Review of Etiology, Diagnosis, and Management Strategies While this usually causes pain in the front of the knee, the compensatory strain on the soft tissues of the inner knee can produce medial discomfort too, especially if the medial retinaculum (the tissue that helps hold the kneecap in place) is being stretched as the patella drifts outward.
Component rotation is best assessed with a CT scan, since plain X-rays are unreliable for measuring rotational alignment. Mild tracking issues sometimes respond to targeted physical therapy focused on strengthening the inner quadriceps muscle, but significant rotational malalignment often requires revision surgery to correct the component position.10PubMed Central. Internal Rotation, Varus, and Anterior Femoral Component Malalignments Adversely Affect Patellofemoral Joint Kinematics in Patellofemoral Arthroplasty
When the Problem Is Partial Knee Replacement
If you had a partial (unicompartmental) replacement rather than a total knee replacement, there is a specific scenario worth knowing about. A medial unicompartmental replacement addresses only the worn inner compartment, leaving the outer compartment and the kneecap groove intact. Over time, arthritis can progress in those untouched areas, and lateral compartment progression is actually the most common reason partial replacements eventually need revision.11PubMed. Lateral unicompartmental knee replacement for the treatment of arthritis progression after medial unicompartmental replacement While that would primarily cause outer knee pain, the altered mechanics of an aging partial replacement can also strain medial structures. Conversely, if you had a lateral partial replacement, new medial compartment wear is a direct cause of inner knee pain. Distinguishing between implant-related problems and disease progression typically requires updated weight-bearing X-rays compared with earlier films.
Persistent Pain, the Nervous System, and Central Sensitization
Sometimes the implant is perfectly positioned, there is no infection, and the soft tissues look fine, yet the inner knee still hurts. In a meaningful fraction of patients, the nervous system itself becomes part of the problem. Persistent post-surgical pain has been reported in roughly 13 to 39 percent of patients at six months to a year, depending on the study, and neuropathic pain (pain originating from nerve dysfunction rather than tissue damage) is present in around 5 to 13 percent at six months.12PubMed Central. Persistent post-surgical pain and neuropathic pain after total knee replacement
Central sensitization is part of this picture. It describes a state in which the spinal cord and brain amplify pain signals, essentially turning up the volume on nerve input that would not normally be painful. Patients with central sensitization before surgery tend to have more severe pain after their knee replacement. A pooled analysis of multiple studies found a moderate-to-large difference in postoperative pain between patients with and without preoperative central sensitization.13PubMed Central. Diagnosis of Central Sensitization and Its Effects on Postoperative Outcomes following Total Knee Arthroplasty: A Systematic Review and Meta-Analysis For these patients, standard treatments like ice, physical therapy, and anti-inflammatory medications may not work well. Medications that calm the nervous system, like gabapentin or duloxetine, along with cognitive behavioral approaches, tend to be more helpful.
Psychological Factors That Amplify Pain
This is not about the pain being “in your head.” Depression, anxiety, and a tendency toward pain catastrophizing (dwelling on the worst possible outcome of a painful sensation) are well-documented predictors of worse pain outcomes after knee replacement. A systematic review and meta-analysis found that catastrophizing, preoperative pain levels, poor mental health, and medical comorbidities had the largest effect sizes for predicting persistent pain.14British Journal of Anaesthesia. Predictors of persistent pain after total knee arthroplasty: a systematic review and meta-analysis A prospective study put a finer point on it: patients with high preoperative catastrophizing scores had roughly five times the odds of persistent pain after surgery compared to those with low scores.15Orthopaedics & Traumatology: Surgery & Research. Psychological factors associated with pain after total knee arthroplasty: a prospective monocentric study
These findings do not mean the pain is imaginary. They mean the brain’s processing of pain signals is heavily influenced by emotional state, and treating only the knee while ignoring depression or catastrophizing leaves a major driver of pain unaddressed. Screening for these factors before surgery and incorporating psychological support into the recovery plan can meaningfully improve outcomes.16PubMed Central. Chronic pain after total knee arthroplasty
Metal Allergy
Knee replacement components are made of metal alloys, most commonly cobalt-chromium and titanium. A small number of patients develop a hypersensitivity reaction to the metal in their implant. Metal sensitization is higher in people with a knee replacement than in the general population and is even higher in patients who have already undergone revision surgery. Symptoms range from unexplained pain and swelling to skin reactions and, in more advanced cases, aseptic loosening of the implant where the bone-implant interface breaks down without any infection.17PubMed Central. Allergy in total knee replacement surgery: Is it a real problem?
Diagnosing metal allergy after knee replacement is frustrating because there is no single definitive test. Patch testing (the same kind used for skin allergies) can suggest sensitization, but a positive patch test does not always mean the implant is causing the symptoms. Blood tests measuring lymphocyte reactivity to specific metals are available, though their accuracy is debated. If metal allergy is strongly suspected and other causes have been excluded, revision to a hypoallergenic implant (often an oxidized zirconium or coated component) is the usual approach.
Vascular Complications Are Rare but Serious
The popliteal artery runs behind the knee, close to the back of the joint capsule. In rare cases, it can be damaged during surgery, leading to a pseudoaneurysm, a pulsating blood-filled bulge in the wall of the artery. Symptoms include unusual and persistent posterior knee pain, swelling, a pulsatile mass in the back of the knee, and sometimes numbness or tingling in the lower leg.18PubMed Central. Do not ignore persistent pain after total knee arthroplasty: Pseudoaneurysm of the popliteal artery after primary total knee arthroplasty
While vascular injury tends to cause pain behind or around the whole knee rather than specifically on the medial side, it belongs in the differential diagnosis because the pain can radiate inward, and because regional anesthesia used after surgery can mask the early warning signs. Vascular complications like pseudoaneurysm or thrombosis may not become obvious for days, and clinicians are cautioned not to attribute concerning symptoms to lingering effects of a nerve block.19PubMed Central. Arterial Complications following Total Knee Arthroplasty (TKA): A Systematic Review and Proposal for Improved Monitoring If your pain is severe, worsening, and accompanied by changes in skin color, temperature, or sensation in the lower leg, get evaluated urgently.
How Doctors Work Through the Diagnosis
Because so many different structures can cause medial knee pain after replacement, diagnosis is usually a process of elimination. Your surgeon will start with a detailed history: when exactly the pain started, whether it is constant or activity-related, what makes it better or worse, and whether you have any numbness, tingling, warmth, or instability. A thorough physical exam can narrow the field considerably. Tenderness right along the joint line suggests a different cause than tenderness a couple of inches below it (which points toward pes anserine bursitis) or a positive Tinel’s sign along the incision (which suggests a neuroma).
Imaging follows. Weight-bearing X-rays check alignment and component positioning. A CT scan can assess component rotation and overhang more precisely. Blood tests, including inflammatory markers like CRP and ESR, screen for infection. Joint fluid aspiration (arthrocentesis) may be needed if infection or crystal arthropathy is suspected.
When standard workup comes back normal and pain persists, nerve blocks can be diagnostic. Genicular nerve blocks, which use local anesthetic injected around specific nerve branches near the knee, can help determine whether the pain is nerve-mediated. In one case report, a patient who got no relief from standard genicular nerve blocks experienced complete pain relief for several hours when a modified protocol targeting additional injection sites was used, suggesting that the standard approach sometimes misses the relevant nerve branches in post-replacement patients.20PubMed Central. Modified genicular nerve diagnostic blocks for persistent pain post total knee arthroplasty: A case report If a diagnostic block confirms nerve-mediated pain, radiofrequency ablation (using heat to disable the nerve branch) is a potential treatment that avoids further surgery on the joint itself.
When to Push for More Answers
Mild inner knee discomfort in the first few months after knee replacement is part of normal healing. The surgical approach runs directly through medial structures, and it takes time for inflammation to settle, incisions to mature, and muscles to rebuild. Most surgeons advise patience during this window, along with diligent physical therapy and appropriate pain management.
The point at which patience should give way to investigation varies, but a reasonable rule of thumb is this: if your medial knee pain is not steadily improving by three to four months, or if it is getting worse at any point, or if it is accompanied by any red flags like fever, wound drainage, severe swelling, or changes in circulation below the knee, a thorough workup is warranted. Do not accept “some people just have pain after knee replacement” as a final answer without the causes discussed above being systematically considered and ruled out. Most of them are treatable once identified, and the frustrating part of post-replacement pain is usually not the treatment itself but the delay in finding the diagnosis.