3.1 Total Hip Arthroplasty (THA): Surgical Approaches, Biomechanics & Component Types

Surgical Corridors and Functional Anatomy in Total Hip Arthroplasty

Total Hip Arthroplasty (THA) is among the most clinically reliable and cost-effective reconstructive procedures in modern orthopaedic surgery. The primary clinical objective of THA is the complete alleviation of arthritic pain, restoration of anatomical joint biomechanics (including horizontal and vertical femoral offset and leg length equality), and re-establishment of functional mobility. The surgical approach chosen by the orthopaedic surgeon dictates the specific intermuscular and internervous planes traversed, the soft-tissue structures detached and repaired, the perioperative neural vulnerabilities, and the specific postoperative movement precautions required.

                    TOTAL HIP ARTHROPLASTY SURGICAL CORRIDORS

       [Posterior / Moore]            [Direct Anterior / DAA]        [Anterolateral / Watson-Jones]
                |                                |                                   |
    - Interval: Gluteus Maximus      - Interval: TFL / Sartorius         - Interval: TFL / Gluteus Medius
      split & short rotators           (True internervous plane)         - Partial abductor release
    - Nerve: Sciatic (peroneal)      - Nerve: Lateral Femoral            - Nerve: Superior Gluteal
    - Highest posterior              - Sparing of abductors/rotators     - Low dislocation rate;
      dislocation risk               - Anterior dislocation risk           risk of Trendelenburg limp

The Posterior (Moore / Southern) Approach

The posterior approach remains one of the most widely utilized surgical exposures globally. The patient is typically positioned in the lateral decubitus position with rigid pelvic positioners.

• Surgical Plane: The incision extends proximally over the posterior border of the greater trochanter and curves posteriorly into the buttock. The gluteus maximus is split along the direction of its muscle fibers (in blunt dissection). The underlying short external rotators—specifically the piriformis, gemelli superior, obturator internus, and gemelli inferior—along with the posterior hip capsule, are detached from their femoral insertions and tagged for subsequent transosseous repair.
• Spared Structures: The gluteus medius and gluteus minimus (the primary abductor complex) and the anterior hip capsule remain completely untouched, preserving functional abductor leverage.
• Neural Vulnerability: The sciatic nerve is at direct risk during exposure and retraction. It exits the pelvis through the greater sciatic foramen inferior to the piriformis muscle. The peroneal division of the sciatic nerve, situated on the posterior-lateral aspect of the nerve bundle, is exceptionally sensitive to traction injury, ischemia from retractor compression against the posterior acetabular wall, or thermal injury from electrocautery. Sciatic nerve injury manifests clinically as foot drop (loss of ankle dorsiflexion and great toe extension) and sensory loss across the dorsal and lateral foot.
• Biomechanical Implications: Because the posterior capsular and muscular restraint is incised, the hip is intrinsically vulnerable to posterior dislocation when placed in positions of combined flexion, adduction, and internal rotation.

The Direct Anterior Approach (DAA / Smith-Petersen / Hueter Interval)

The direct anterior approach has gained substantial popularity due to its muscle-sparing characteristics. The patient is placed supine, frequently on a specialized orthopaedic traction table (e.g., Hana table) that facilitates hyperextension, adduction, and external rotation of the operative limb for femoral preparation.

• Surgical Plane: The DAA utilizes the interval between the tensor fasciae latae (TFL) laterally (innervated by the superior gluteal nerve) and the sartorius / rectus femoris medially (innervated by the femoral nerve). This represents a true internervous and intermuscular plane. No muscles or tendons are detached from the pelvis or femur during routine primary cases, although the anterior capsule is resected or incised.
• Neural Vulnerability: The lateral femoral cutaneous nerve (LFCN) is exquisitely vulnerable during superficial dissection. The LFCN passes beneath the inguinal ligament near the anterior superior iliac spine (ASIS) and branches across the proximal anterior thigh. Traction or transection of the LFCN produces meralgia paresthetica, characterized by dysesthesia, burning pain, and numbness over the anterolateral aspect of the thigh. Deep anterior retractors placed too aggressively over the medial acetabular wall can also compress or injure the femoral nerve or femoral vascular bundle.
• Biomechanical Implications: Sparing the posterior capsule and abductors results in excellent intrinsic resistance to posterior displacement. However, the compromised anterior capsule makes the joint vulnerable to anterior dislocation if forced into extreme hyperextension and external rotation.

The Anterolateral (Watson-Jones) and Direct Lateral (Hardinge) Approaches

• Anterolateral Approach: Exploits the plane between the tensor fasciae latae (TFL) and the anterior border of the gluteus medius. It requires partial elevation or detachment of the anterior third of the gluteus medius and minimus tendons from the greater trochanter.
• Direct Lateral Approach (Hardinge / Transgluteal): Involves a longitudinal split through the gluteus medius and vastus lateralis in continuity, providing panoramic visualization of both the acetabulum and proximal femur.
• Neural Vulnerability: The superior gluteal nerve traverses between the gluteus medius and minimus, approximately 3 to 5 cm proximal to the greater trochanter tip. Incisions or retractor placement extending more than 5 cm proximal to the trochanter risk denervating the anterior portion of the gluteus medius, gluteus minimus, and TFL.
• Biomechanical Implications: Both approaches offer exceptional posterior stability with remarkably low dislocation rates. However, detachment, splitting, or denervation of the gluteus medius can result in persistent postoperative abductor weakness, manifested clinically as an uncompensated or compensated Trendelenburg lurch (pelvic drop toward the contralateral swing limb during single-leg stance).

Comparative Summary of Surgical Approaches

Biomechanics, Component Fixation, and Tribology

A total hip prosthesis replaces the diseased native articulation with an acetabular component (shell and liner) and a femoral component (stem and modular head). The long-term survivorship of the implant depends on mechanical stability, biological fixation, and the wear kinetics of the bearing surfaces (tribology).

Biological Press-Fit vs. Cemented Fixation

                      IMPLANT FIXATION MODALITIES

    [Uncemented / Press-Fit Fixation]        [Cemented / PMMA Fixation]
    - 1-2 mm under-reaming interference fit   - Mechanical interlock / grouting agent
    - Porous coating / trabecular titanium    - Rapid full weight-bearing stability
    - Requires <150 µm micromotion for        - Indicated in Dorr Type C osteopenic bone
      osteointegration                        - Risk: Bone Cement Implantation Syndrome

1. Uncemented (Press-Fit / Porous Ingrowth) Fixation:

   • Mechanism: The acetabular shell and femoral stem feature porous surfaces (e.g., sintered beads, titanium plasma spray, porous trabecular metal/tantalum, or hydroxyapatite coating). The bone is prepared with precision reamers and broaches to achieve an interference fit (typically under-reamed by 1–2 mm relative to the implant diameter).
   • Biology: Primary stability is achieved mechanically by initial frictional interference. Secondary biological stability occurs through osteointegration (new bone growing into the microscopic pores). For successful ingrowth, initial interfacial micromotion must remain strictly below 40 to 150 microns; excessive initial micromotion (>150 microns) results in fibrous tissue ingrowth rather than osteointegration, leading to early aseptic loosening.
   • Indications: Standard choice for younger, active patients and patients with good baseline bone mineral density (Dorr Type A and B bone geometry).

2. Cemented Fixation (Polymethylmethacrylate - PMMA):

   • Mechanism: PMMA bone cement is not an adhesive or glue; it functions as a mechanical grouting agent that interlocks within the micro-trabeculae of cancellous bone and distributes load evenly across the bone-implant interface.
   • Indications: Indicated in patients with severe osteoporosis, wide "stovepipe" femoral canals with thin cortices (Dorr Type C bone), irradiated bone, or elderly patients requiring immediate, rigid mechanical stability to minimize peri-prosthetic fracture risk.
   • Bone Cement Implantation Syndrome (BCIS): A life-threatening perioperative complication occurring during cement pressurization and stem insertion. The injection of monomer under high pressure forces intramedullary contents (marrow fat, air microemboli, bone fragments, and vasoactive mediators) into the femoral venous system and right atrium. This triggers pulmonary vasoconstriction, acute right ventricular strain, severe systemic hypotension, hypoxemia, cardiac dysrhythmias, and potential sudden cardiovascular collapse.
   • Nursing & Anesthesia Vigilance for BCIS: Ensuring aggressive pre-cementation intravascular volume loading, delivering 100% FiO2 during cement pressurization, continuous arterial line monitoring, and having immediate vasopressors (phenylephrine, epinephrine) primed at the bedside.

Bearing Surfaces and Tribology (Wear Physics)

The choice of articulation interface determines the generation of microscopic particulate wear debris, which is the primary driver of macrophage-mediated periprosthetic osteolysis and aseptic loosening.

• Metal-on-Highly Cross-Linked Polyethylene (MoP): Cobalt-chromium (CoCr) femoral head articulating with highly cross-linked ultra-high-molecular-weight polyethylene (HXLPE). Cross-linking via gamma irradiation drastically reduces volumetric wear rates (<0.01 mm/year compared to >0.1 mm/year in legacy non-cross-linked polyethylene).
• Ceramic-on-Highly Cross-Linked Polyethylene (CoP): Ceramic head (alumina matrix composite, e.g., Biolox Delta) on HXLPE. Offers exceptional surface wettability, lower friction coefficients, and zero risk of metallic ion release. Currently the most common bearing combination in primary THA.
• Ceramic-on-Ceramic (CoC): Lowest volumetric wear rate of all bearing couples. However, drawbacks include brittle fracture risk (rare in modern composites), component chipping during insertion, and audible "squeaking" during articulation.
• Metal-on-Metal (MoM): Largely historical/abandoned due to adverse local tissue reactions (ALTR), pseudotumors, and elevated serum cobalt and chromium ion levels leading to systemic metallosis.

Dual-Mobility Acetabular Technology

Originally designed by Gilles Bousquet in France, dual-mobility cup designs provide two distinct articulation interfaces within the same acetabular shell:

1. An inner bearing: A small femoral head (typically 22 mm or 28 mm) articulates within a mobile polyethylene liner.
2. An outer bearing: The large mobile polyethylene liner articulates within a smooth, polished metal acetabular shell.

                       DUAL-MOBILITY BEARING MECHANICS

            [Outer Metal Shell] (Fixed in native acetabulum)
                     |
            [Mobile Polyethylene Liner] (Outer Articulation Interface)
                     |
            [Small Femoral Head] (Inner Articulation Interface)
                     |
            [Femoral Stem]

    * Primary low-demand motion occurs at inner interface (head inside liner).
    * High-demand motion recruits outer interface, dramatically increasing the
      effective head diameter and required displacement ("jump distance").

• Biomechanics: Dual-mobility systems exponentially increase the "jump distance" (the vertical displacement that the center of the femoral head must travel before completely dislocating over the acetabular rim). Primary low-demand motion occurs at the inner interface, while terminal high-demand motion recruits the outer interface.
• High-Risk Indications for Dual Mobility:
  • Neuromuscular disorders predisposing to recurrent dislocation (Parkinson's disease, post-stroke hemiparesis, cerebral palsy).
  • Patients with rigid spinopelvic alignment (e.g., prior extensive lumbar spinal fusions), in whom loss of lumbar lordosis alters pelvic tilt and exposes the hip to impingement.
  • Cognitive impairment, severe dementia, or history of non-compliance with postop movement restrictions.
  • Revision THA for recurrent mechanical instability.
  • Severe abductor mechanism deficiency.

Clinical Nursing Surveillance and Assessment Protocol

Postoperative nursing surveillance of the primary total hip arthroplasty patient requires an integrated understanding of the surgical approach, component design, and underlying patient comorbidities.

Comprehensive Neurovascular Examination Protocol

A neurovascular check must be executed systematically and compared bilaterally immediately upon PACU arrival and at scheduled postoperative intervals:

• Peroneal Nerve (L5): Sensation over the first dorsal web space and dorsal foot; motor function tested by active ankle and great toe dorsiflexion against resistance.
• Tibial Nerve (S1): Sensation on the plantar surface of the foot; motor function tested by active plantarflexion ("step on the gas pedal").
• Femoral Nerve (L2–L4): Sensation over the anterior thigh and medial leg; motor function tested by active quadriceps contraction (straight leg raise or isometric quad set).
• Vascular Assessment: Palpation of dorsalis pedis and posterior tibial pulses (grade 0 to 4+), capillary refill time (<3 seconds), and skin temperature/color. Any asymmetry or pulse deficit warrants immediate Doppler ultrasound verification and surgical team notification.