Knee Replacement Surgery: Muscle Disruption, Affected Tissues, and Recovery

What muscles are damaged during knee replacement?

While the primary goal of knee replacement surgery is joint reconstruction, some degree of muscle disruption is an unavoidable consequence, primarily affecting the quadriceps femoris group—especially the vastus medialis—due to the necessary surgical incision and retraction for joint access.

Understanding Knee Replacement Surgery (Arthroplasty)

Total Knee Arthroplasty (TKA), commonly known as knee replacement surgery, is a major orthopedic procedure designed to relieve pain and restore function in a severely damaged knee joint. This involves removing damaged bone and cartilage from the thigh bone (femur), shin bone (tibia), and kneecap (patella), and replacing them with prosthetic components made of metal alloys, high-grade plastics, and polymers. To access the joint, surgeons must navigate through various layers of soft tissues, including skin, fascia, and muscle, which inevitably leads to some degree of tissue disruption.

The Primary Surgical Approach and Muscle Disruption

The most common surgical technique for TKA is the medial parapatellar approach. This approach involves making an incision along the inner side of the kneecap (patella) and then retracting the patella laterally to expose the knee joint. This necessary access directly impacts the extensor mechanism of the knee.

The Quadriceps Femoris Muscle Group

The quadriceps femoris is a powerful group of four muscles located on the front of the thigh, crucial for knee extension. All four heads converge into the quadriceps tendon, which attaches to the patella. During a medial parapatellar approach, the following are most directly affected:

• Vastus Medialis: This is arguably the most directly impacted muscle. The incision often passes through or along the border of the vastus medialis, particularly its distal fibers known as the vastus medialis obliquus (VMO). The VMO plays a critical role in patellar tracking and the last degrees of knee extension. Damage here can contribute significantly to post-operative quadriceps weakness and patellar instability.
• Rectus Femoris, Vastus Lateralis, and Vastus Intermedius: While these muscles are typically not incised directly in the medial parapatellar approach, they are subjected to significant retraction forces during surgery. This retraction can cause micro-trauma, stretching, and temporary denervation, contributing to post-operative pain, swelling, and inhibited muscle activation.

The Patellar Tendon

While not a muscle, the patellar tendon is a crucial soft tissue structure that connects the patella to the tibia. It is an extension of the quadriceps tendon. During surgery, the patellar tendon is meticulously protected, but its close proximity to the surgical field means it is subject to manipulation and the forces of retraction, which can lead to post-operative tenderness and swelling.

Other Soft Tissues Affected (Beyond Primary Muscles)

Beyond the direct muscle disruption, several other soft tissues are necessarily incised, stretched, or manipulated during knee replacement:

• Joint Capsule: The fibrous capsule surrounding the knee joint is incised to gain access to the articular surfaces.
• Ligaments: The medial collateral ligament (MCL) is often released or partially detached to allow for proper joint alignment and component placement, though it is typically re-tensioned or repaired. The anterior and posterior cruciate ligaments (ACL and PCL) are usually resected (removed) in total knee replacements, as the prosthetic components replace their function.
• Retinacula: The medial and lateral patellar retinacula, which are fibrous expansions supporting the patella, are incised or stretched.
• Synovial Membrane: The inner lining of the joint capsule is disrupted.
• Bursae: Any inflamed or problematic bursae around the knee may be removed.

Indirect Effects and Secondary Weakness

Even muscles not directly incised can experience secondary weakness due to the surgical trauma, pain, swelling, and subsequent immobilization or reduced activity:

• Hamstrings: While not directly cut in most TKA approaches, the hamstrings can suffer from disuse atrophy and altered recruitment patterns post-surgery. Their role in knee flexion and stability is crucial for balanced gait and recovery.
• Gluteal Muscles: Weakness in the hip abductors and extensors (gluteus medius, gluteus maximus) is common after TKA due to altered gait mechanics, pain avoidance, and general deconditioning. This can impact overall lower limb biomechanics and stability.
• Calf Muscles: Reduced weight-bearing and activity can lead to atrophy and stiffness in the calf musculature, potentially impacting ankle mobility and venous return.

Surgical Techniques Aimed at Muscle Preservation

Advances in surgical techniques aim to minimize soft tissue damage:

• Quadriceps-Sparing Approaches: Some surgeons utilize less invasive techniques, such as the subvastus or midvastus approaches, which attempt to avoid incising the vastus medialis directly. Instead, they try to dissect around or split the muscle fibers. While these approaches may offer some benefits in early recovery, they still involve significant soft tissue manipulation and retraction, and the long-term functional benefits compared to the traditional medial parapatellar approach are still debated.

The Path to Recovery: Rehabilitation and Muscle Restoration

Despite the unavoidable muscle and soft tissue disruption, dedicated post-operative rehabilitation is crucial for restoring function. Physical therapy focuses on:

• Pain and Swelling Management: To facilitate early movement.
• Regaining Range of Motion: Essential for daily activities.
• Strengthening: Targeting the quadriceps (especially the VMO), hamstrings, and gluteal muscles to restore balance and power.
• Proprioception and Balance Training: To improve neuromuscular control and reduce fall risk.
• Gait Retraining: To normalize walking patterns.

While muscle damage occurs during knee replacement, the human body's capacity for healing, combined with expert surgical techniques and diligent rehabilitation, allows most individuals to regain significant strength and function, ultimately achieving the goal of reduced pain and improved quality of life.