Back of Knee Pain

Written by: Dr. Samantha Smith PT, DPT (Physical Therapist & Knee Replacement Specialist)

Knee replacement surgery, also known as knee arthroplasty, is a common orthopedic procedure performed to alleviate chronic knee pain and restore joint function in individuals with severe knee damage, typically caused by osteoarthritis, rheumatoid arthritis, or traumatic injury. The most common reason individuals need a knee replacement is due to knee arthritis typically due to a previous injury earlier in life. The procedure involves removing the cartilage, bone and some ligaments (ACL and/or PCL) from the knee joint and replacing them with artificial components made of metal and polyethylene AKA plastic. There are two primary types of knee replacements: total knee replacement (TKR), where the entire joint is resurfaced, and partial knee replacement (PKR), where only the affected compartment is addressed. The most common partial knee replacement is a medial partial knee replacement, which involves replacing only the inner condyles of the femur and inner surface of the tibia. While knee replacement surgery is highly effective in reducing pain and improving mobility, some patients may experience continued or new-onset pain behind the knee. This discomfort can stem from several sources, including Baker’s cysts caused by excess joint fluid, hamstring, popliteal or gastrocnemius tendonitis from weakness or poor mechanics, nerve irritation due to surgical trauma and/or a drastic change in biomechanics from obtaining a knee replacement. Additionally, prosthetic malalignment or muscular imbalances may place undue stress on posterior structures, leading to chronic discomfort and pain. Understanding the potential causes and targeted management strategies for back of knee pain is crucial for optimizing recovery and ensuring long-term functional outcomes after knee replacement surgery.

Common Reasons for Back of Knee Pain

Baker’s Cyst (Popliteal Cyst):

Following a total knee replacement surgery or partial knee replacement surgery, joint inflammation or increased synovial fluid production can lead to the formation of a Baker’s cyst. This fluid-filled sac develops behind the knee in the popliteal bursa and can cause a feeling of tightness, swelling, and discomfort. The cyst forms as a response to increased pressure or inflammation in the knee joint. A Baker’s cyst is a secondary response to an underlying joint issue. Many knee replacement patients experience a Baker’s cyst prior to surgery due to inflammation in the joint from knee arthritis. A Baker’s cyst is likely to linger after a knee replacement surgery and often times, the cyst comes back even if it has been removed by the surgeon.

Some common symptoms of a Baker’s cyst are swelling behind the knee, stiffness, and a palpable lump that may fluctuate in size. Pain may worsen with knee flexion or prolonged standing. A Baker’s cyst is diagnosed using patient history, MRI and/or ultrasound. 

After a knee replacement, it is not uncommon for the Baker’s cyst to linger for months and months. Most patients will see an increase in symptoms of the cyst during the first 3 months of their knee replacement recovery. Overtime, the Baker’s cyst begins to fluctuate in size until it slowly goes away. The cyst will increase in size and discomfort will increase if the patient over works the leg muscles. It is key to continue strengthening the leg appropriately with a gentle, progressive exercise program to help reduce the cyst. Compression, ice, and anti-inflammatory medications can also be useful in the short term to manage symptoms. In severe cases, aspiration or surgical excision can also be used.
 

Hamstring Tendonitis/Tendinopathy

The hamstring tendons attach near the back of the knee. Surgical manipulation, muscle imbalance, or altered gait patterns post-surgery can place additional stress on these tendons, leading to inflammation, overuse of the tendons, and/or micro tears of the tissues. During a knee replacement, the tissues of the knee are highly irritated due to the nature of the a knee replacement surgery. The surgeon often checks the “balancing” of the muscles, tendons and ligaments during the surgery by fully bending and straightening the knee. This can cause a lot of issues post surgery. For knee replacement patients who have knee valgus or varus (knock knee or bow leg) formation, you may have more tissue pain post surgery as well. For patients who have not had full knee range of motion for years prior to surgery, you also may have more tissue pain post surgery due to the manipulation during surgery. 

After surgery, it is also very common to have muscle weakness in the hamstrings. Although the quad muscles tend to be weakened more, the hamstrings also experience significant weakness. Weak muscles can cause pain, stiffness and compensations. Our bodies adjust to weakness by using other muscles to pick up the slack. This can cause overuse injuries and inflammation. It is important for our muscles to all work together to reduce pain. A generalized exercise program can help with this. Focusing on strengthening the common muscles weakened after a knee replacement can reduce overall pain. If you are looking for a specialized exercise program, the 4 Week Strength, Balance & Stretching Class by Dr. Samantha Smith PT can be very beneficial post total knee replacement or partial knee replacement. 

Popliteal Tendonitis

Similar to hamstring tendonitis, the popliteus muscle and tendon can get very irritated after a knee replacement. Surgical trauma, altered joint mechanics, or excessive stress can lead to inflammation of this structure. There is going to be excessive stress on all the muscles of the knee after a knee replacement due to weakness in the leg. Weak muscles cause the tendons to become overloaded. Overloaded tendons are very painful! This is why it is so important to strengthen your leg prior to surgery. If you have a knee replacement coming up in the future, focus on those big muscle groups of the legs (quads, hamstrings, calves, glutes). 

Popliteal tendonitis causes deep, localized pain at the back of the knee, particularly with knee flexion and extension. Pain may increase with walking downhill or when transitioning from sitting to standing. Rest, targeted strengthening exercises, manual therapy, and ice application may help reduce inflammation of the tendon to reduce pain. 

 
Nerve Impingement or Neuropathy

During a total knee replacement surgery, nerves will be irritated due to the nature of a knee replacement surgery. Orthopedic surgeons do all they can to avoid nerve irritation, but unfortunately, there is always going to be some irritation to the nerves. Nerves can be compressed by surgical tools or nerve can be stretched due to the realignment of the knee. It is not often nerves are cut or injured during a knee replacement surgery. This would cause significant muscle weakness, nerve pain and disability. An experienced surgeon will avoid severe injury to nerves at all costs. Nerves that radiate to the back of the knee would be the sciatic nerve, tibial nerve, and peroneal nerve. The sciatic nerve is often bothered by the positioning of the patient on the table and the post operative recovery. Altered gait mechanics can cause a flare up of sciatica, especially if the patient has not completed a pre-surgery exercise program to help strengthen the muscles to help support your back. 

Nerve glides can also be helpful to reduce nerve pain. Nerve glides stretch and move the nerves causing a relief in symptoms for some. 

 
Gastrocnemius Strain or Injury

The gastrocnemius (calf) muscle originates just above the knee and extends to the Achilles tendon. Postoperative muscle imbalances or compensatory gait patterns can strain this muscle, causing pain behind the knee. Common symptoms of a calf strain include aching or cramping at the back of the knee, which can increase by walking, standing, or climbing stairs. Massage and heat can also help in the short term to reduce symptoms. Continuing to exercise & stretch can help revolve the strain. Isometric and eccentric muscle contractions of the calf muscles can sometimes immediately reduce muscle aching. Since back of the knee pain is so common post knee replacement surgery, the 4 Week Strength, Balance & Stretching Class specializes in helping reduce these common pains. This class will teach you how to complete isometric and eccentric muscle contractions for the calf (and the hamstrings, quads and glutes). 

Deep Vein Thrombosis (DVT)

Another rare, but noteworthy, cause of back of the knee pain can be from a blood clot or deep vein thrombosis (DVT). A blood clot can form in the deep veins of the leg after surgery due to immobility, inflammation, or vascular injury. It is a serious complication requiring immediate medical attention. Blood clots usually occur within the first 14 days post knee replacement surgery, but can occur after this timeframe as well. Some patients after a knee replacement may be more prone to blood clots due to medical history. Often times, orthopedic surgeons give blood thinners post surgery to mitigate the risk of DVTs. Common symptoms of a DVT are swelling, warmth, redness, and a deep, throbbing pain behind the knee or calf. Pain may intensify with dorsiflexion (positive Homan’s sign). It is hard to diagnose a DVT since most knee replacement patients have these symptoms regardless of a blood clot or not. This is why it is important to mention these issues to your healthcare team. 

If the surgeon believes a DVT is the cause of your back of the knee pain, they will likely send you to the emergency room or a vascular clinic for a thorough assessment. An ultrasound will be completed and possibly a blood test to test for elevated D-dimer. 

One of the best ways to reduce the risk of blood clots is to keep moving! Physical therapy after a knee replacement can significantly reduce the risk of a blood clot. Receiving professional guidance after a total knee replacement is very important. We do not want to over exercise or under exercise. If you need more guidance with your knee replacement recovery, the Post Surgery Course can be a useful step by step guide to keep on you track. 

Prosthetic Malalignment or Loosening

A rare occurrence, but something to consider, would be the implant itself becoming loose and causing pain at the back of the knee. A loose implant will present with symptoms of new onset clicking at the knee, persistent instability, sudden increased stiffness, sudden increased pain, and/or a clunking or shifting feeling in the knee. Typically, loosening of the implant prior to 5 years after the replacement is not common. This can be diagnosed via xray by your surgeon. If the implant is loose, a revision would be needed. 

Improper positioning of the prosthetic components can alter joint mechanics, placing excessive stress on posterior structures. This would be different than loosening of the implant. If the implant is not placed in the knee correctly, ongoing pain may occur. A second opinion from a new surgeon would be wise to obtain to make sure the original surgeon has not overlooked anything.