Knee Replacement: Muscles and Tissues Affected, Surgical Approaches, and Recovery

What muscles are cut during knee replacement?

During a total knee replacement (arthroplasty), the primary goal is to replace damaged joint surfaces while preserving as much healthy tissue as possible. While no major muscle bellies are typically "cut" across their entire width, specific tendons, retinacula, and the joint capsule are incised or manipulated to access the joint.

Understanding Total Knee Arthroplasty (TKA)

Total knee arthroplasty (TKA) is a common surgical procedure performed to alleviate pain and restore function in individuals with severe knee arthritis. The procedure involves removing damaged bone and cartilage from the thigh bone (femur), shin bone (tibia), and kneecap (patella), and replacing them with artificial components made of metal and plastic. To achieve this, surgeons must carefully navigate the complex anatomy surrounding the knee joint.

Key Muscles and Tissues Affected During Knee Replacement

The specific tissues affected can vary slightly depending on the surgical approach, but the most common technique is the medial parapatellar approach. This approach involves an incision on the front of the knee, medial (inside) to the kneecap.

Here's a breakdown of the structures typically incised or significantly manipulated:

• Skin and Subcutaneous Tissue: The initial incision is made through the skin and the layer of fat beneath it.
• Joint Capsule and Synovium: The fibrous capsule surrounding the knee joint and its inner lining (synovium) are incised to gain access to the joint space.
• Quadriceps Tendon Expansion and Vastus Medialis Obliquus (VMO): To allow the patella (kneecap) to be moved out of the way (eversion) for full joint exposure, the medial aspect of the quadriceps tendon and the underlying fibers of the vastus medialis obliquus (VMO) muscle are often incised. This incision is typically longitudinal (along the length of the fibers) or a partial release, rather than a complete transection of the muscle belly.
• Medial Patellar Retinaculum: This fibrous band, which helps stabilize the patella on the medial side, is incised as part of the medial parapatellar approach.
• Menisci: Both the medial and lateral menisci, which are C-shaped cartilage shock absorbers, are removed as part of the joint resurfacing.
• Cruciate Ligaments:
  • Anterior Cruciate Ligament (ACL): The ACL is routinely removed during TKA.
  • Posterior Cruciate Ligament (PCL): Depending on the implant design (PCL-retaining vs. PCL-substituting) and surgeon preference, the PCL may be preserved or removed.
• Collateral Ligaments (MCL and LCL): The Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL) are crucial for knee stability and are preserved whenever possible. However, in cases of significant deformity (e.g., severe bowleg or knock-knee), the MCL (often its deep fibers) or, less commonly, the LCL may require a partial release to balance the soft tissues and ensure proper alignment of the new joint. This is a release, not a full cut across the ligament.
• Hamstring Tendons: The tendons of the hamstring muscles (semitendinosus, semimembranosus, biceps femoris) are generally not cut during TKA. They may be retracted to allow access to the posterior aspect of the joint, but their integrity is maintained.

Surgical Approaches and Muscle Sparing Techniques

While the traditional medial parapatellar approach involves some incision of the quadriceps tendon expansion and VMO, newer techniques aim to minimize tissue disruption:

• Minimally Invasive Approaches: These approaches use smaller incisions and attempt to reduce the amount of soft tissue disruption. Examples include:
  • Mid-Vastus Approach: Involves splitting the VMO muscle fibers longitudinally rather than detaching them, potentially causing less disruption.
  • Sub-Vastus Approach: Bypasses the VMO entirely by going underneath it, though this approach can be more technically challenging and may not be suitable for all patients.
• Quadriceps-Sparing Approaches: These are variations designed to reduce trauma to the quadriceps mechanism, which is critical for post-operative recovery and strength. The goal is always to preserve as much muscle and tendon integrity as possible.

Despite these advancements, some degree of soft tissue manipulation is always necessary to expose the joint adequately for component implantation.

The Impact on Muscle Function and Rehabilitation

The incisions and manipulations during TKA, particularly to the quadriceps mechanism, inevitably lead to:

• Post-operative Weakness: The quadriceps muscles, which are vital for knee extension and weight-bearing, are often the most affected. This weakness is a primary reason for the initial difficulty in walking and performing daily activities after surgery.
• Pain and Swelling: The surgical trauma causes inflammation, pain, and swelling, further inhibiting muscle function.
• Scar Tissue Formation: As tissues heal, scar tissue forms, which can sometimes restrict movement or lead to stiffness if not managed with rehabilitation.

Rehabilitation is paramount after knee replacement surgery. A structured physical therapy program focuses on:

• Restoring Range of Motion: Early, controlled movement prevents stiffness.
• Strengthening: Progressive exercises to rebuild quadriceps and hamstring strength, as well as hip and core stability.
• Gait Training: Re-educating proper walking patterns.
• Pain Management: To allow participation in therapy.

Consistent adherence to a rehabilitation program is critical for regaining full function and optimizing the long-term success of the knee replacement.

Recovery and Long-Term Considerations

The recovery period after knee replacement typically involves several weeks to months of intensive rehabilitation. While the initial pain and swelling subside, regaining full muscle strength and endurance can take up to a year or even longer. Patients are encouraged to engage in regular, low-impact activities to maintain joint health and muscle strength. The goal is to return to a functional, active lifestyle with reduced pain, understanding that the journey involves dedicated effort in regaining the strength and mobility impacted by the surgical intervention.

Conclusion

While the term "cut" might suggest extensive muscle damage, knee replacement surgery is designed to be as minimally invasive as possible while achieving the necessary joint exposure. The primary muscular structures directly incised are typically the medial aspect of the quadriceps tendon and the underlying VMO fibers, along with the joint capsule and retinacula. Major muscle bellies are generally retracted rather than severed. The focus on preserving vital ligaments (MCL, LCL) and the intensive post-operative rehabilitation underscore the commitment to restoring optimal knee function and stability after this transformative procedure.