4.3 Surgical Anatomy of the Musculoskeletal System

Surgical Anatomy of the Musculoskeletal System

Quick Answer: Mastery of the musculoskeletal system involves understanding bone architecture, the biomechanics of joints, the stabilizing roles of ligaments, and the exact anatomical landmarks used during major orthopedic reconstructions like total hip and knee arthroplasties.

Bone Structure and Physiology

Bones form the rigid framework of the body, providing support, protection, and a leverage system for muscular action. A typical long bone (e.g., femur, humerus) consists of a cylindrical shaft called the diaphysis and two expanded ends called the epiphyses. The metaphysis is the transitional zone between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in children, which ossifies into the epiphyseal line in adults.

The outer layer of bone is composed of dense, solid cortical (compact) bone, providing strength and rigidity. The interior, particularly at the epiphyses, consists of cancellous (spongy or trabecular) bone, which forms a porous network that houses red bone marrow. The medullary cavity within the diaphysis contains yellow bone marrow, which is primarily adipose tissue.

A tough, vascular, fibrous membrane called the periosteum covers the entire external surface of the bone, except at the articular surfaces where hyaline cartilage is present. The periosteum is deeply anchored to the bone by Sharpey's fibers. It plays a critical role in bone growth, repair, and nutrition. During orthopedic surgery, the periosteum must be carefully incised and elevated (using a periosteal elevator like a Key or Cobb) to expose the bone while preserving its osteogenic potential for postoperative healing.

Joints and Articulations

Joints (articulations) are classified functionally by their degree of movement (synarthroses, amphiarthroses, diarthroses) and structurally by the material binding them (fibrous, cartilaginous, synovial).

Synovial joints are the most common and the most surgically relevant, as they provide free movement. Key features of a synovial joint include:

• Articular Cartilage: A layer of hyaline cartilage covering the opposing bone surfaces, absorbing shock and reducing friction.
• Joint Capsule: A double-layered enclosure consisting of an outer fibrous capsule (providing stability) and an inner synovial membrane.
• Synovial Fluid: A viscous fluid secreted by the synovial membrane that lubricates the joint and nourishes the avascular articular cartilage.
• Ligaments and Menisci: Reinforcing bands of tissue and fibrocartilaginous pads (e.g., in the knee) that improve the fit between bone ends and stabilize the joint.

Soft Tissues: Ligaments and Tendons

While ligaments and tendons are both composed of dense regular connective tissue, their functions are distinct:

• Tendons connect muscle to bone. They transmit the mechanical force of muscle contraction to the skeletal system. An example is the Achilles tendon, connecting the calf muscles to the calcaneus. In surgery, ruptured tendons must be repaired with strong, non-absorbable sutures (e.g., Ethibond, FiberWire) using specialized locking techniques like the Krackow stitch.
• Ligaments connect bone to bone. They are crucial for joint stability, limiting movement to normal planes and preventing excessive translation. The Anterior Cruciate Ligament (ACL) of the knee is frequently torn during sports and is often reconstructed using an autograft (e.g., bone-patellar tendon-bone or hamstring) or allograft.

Surgical Anatomy for Total Joint Arthroplasty

Total joint arthroplasty (TJA) involves replacing the diseased articular surfaces of a joint with prosthetic components. Precise knowledge of anatomical landmarks is required to restore normal biomechanics and leg length.

Total Hip Arthroplasty (THA)

The hip is a ball-and-socket synovial joint formed by the femoral head and the acetabulum of the pelvis. During THA, the surgeon exposes the hip joint via an anterior, anterolateral, or posterior approach. The posterior approach is very common. Key landmarks include the greater trochanter, the femoral neck, and the acetabular rim. The femoral neck is resected at a specific angle and level above the lesser trochanter. The acetabulum is reamed to remove remaining cartilage down to bleeding subchondral bone, and a hemispherical shell is impacted. A major neurovascular structure at risk during the posterior approach is the sciatic nerve, which exits the pelvis through the greater sciatic foramen and runs posterior to the short external rotator muscles. Retractors must be placed carefully to avoid compressing or stretching this nerve.

Total Knee Arthroplasty (TKA)

The knee is a complex hinge joint consisting of the tibiofemoral and patellofemoral articulations. Stability relies heavily on the collateral ligaments (MCL and LCL) and cruciate ligaments (ACL and PCL). During a TKA, the diseased distal femur, proximal tibia, and posterior patella are resected using precise cutting blocks and jigs to ensure correct alignment.

• The distal femur is prepared with anterior, posterior, chamfer, and distal cuts.
• The proximal tibia is resected perpendicular to its mechanical axis. Depending on the implant design, the PCL may be preserved (Cruciate Retaining or CR knee) or sacrificed and compensated for by a cam-and-post mechanism in the implant (Posterior Stabilized or PS knee). Soft tissue balancing—ensuring the MCL and LCL have equal tension in both flexion and extension—is absolutely critical for the long-term success of the prosthesis.