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Hip

Alistair Macey, Peter Young

Introduction

The hip is a ball and socket synovial joint between the pelvis and femur (Fig. 2.1). The acetabulum is formed by the fusion of the triradiate cartilage between the three ossification centres of the pelvis: ilium, ischium and pubis. It is further deepened by the labrum and surrounded by a strong capsule with three ligamentous condensations (iliofemoral, ischiofemoral and pubofemoral ligaments) (Fig. 2.1). The iliofemoral ligament in particular is one of the strongest ligaments in the body and is tight in hip extension to reduce energy expenditure during stance. The acetabulum is anteverted 15° and covers the femoral head at an angle of 45°. An important landmark in elective practice is the transverse acetabular ligament, which represents the inferior portion of the acetabular labrum. It has a constant alignment in nondysplastic hips and can provide a useful guide to acetabular implant orientation (Table 2.1).

Fig. 2.1 Hip joint anatomy. The capsule follows the intertrochanteric crest anteriorly and incorporates the iliofemoral ligament. Posteriorly the capsule attaches part way up the femoral neck and incorporates the ischiofemoral ligament. This is important in hip preservation surgery as the retinacular vessels penetrate the femoral head through the capsular attachment. The transverse acetabular ligament is a constant anatomical structure that can aide implant alignment during arthroplasty.

Table 2.1

Radiological Evaluation of Hip Dysplasia Measure What is Evaluated How It's Measured Acetabular Index Femoral head coverage Line through the tear drops (Hilgenreiner's line) transecting a line from teardrop to superolateral rim of acetabulum. Normal <35° Centre-Edge Angle Femoral head coverage and subluxation Vertical line from centre of head to superior apex of femoral head and oblique line from centre of head. Normal >25°, borderline 20-25°, abnormal <25° Crossover Sign Retroversion The anterior wall should be medial to the posterior wall. A retroverted acetabulum will result in crossing of the walls and a prominent ischial spine. Posterior Wall Sign Anteversion The posterior wall of the acetabulum should be medial to the centre of the femoral head. If it lies laterally, this is indicative of an anteverted acetabulum. a-Angle Femoral offset Line along centre of femoral neck to centre of head transecting line from superior head-neck junction to centre of head. A value >60° signifies "cam impingement"

The femoral head is predominantly covered with a cartilage cap to permit a large range of movement. The hip capsule attaches anteriorly along the intertrochanteric crest and posteriorly part way up the femoral neck. The blood supply to the femoral head penetrates through the capsular attachment, which is important in hip pathology such as femoral neck fracture or avascular necrosis. In the adult the blood supply is predominantly through the medial femoral circumflex artery, which reaches the capsule through the quadratus muscle and must be protected during joint preserving surgery through a posterior approach. The normal femoral neck is anteverted 15° with a neck-shaft angle of 125°. The greater and lesser trochanters are bony protuberances from the proximal femur that permit numerous muscle attachments around the proximal femur, predominantly from muscles originating around the pelvis.

Adult Manifestations of Hip Dysplasia

Clinical Summary

Developmental dysplasia of the hip in childhood (Chapter 10) can result in long-term acetabular and/or femoral anatomical abnormalities. The presence of dysplastic features on imaging is associated with a four-fold increase in the development of osteoarthritis. Dysplasia can affect both the acetabulum and femur.

A reduced joint contact area is responsible for point loading and reduced lubrication. This culminates in loading and accelerated chondral and labral damage. Patients commonly present with onset of symptoms in their third to fifth decade of life.

Symptoms

Symptoms arise from progressive degeneration and tearing of the anterosuperior labrum followed by chondral surface and eventually degenerative joint disease. There may be an associated femoroacetabular impingement (FAI) due to reduced femoral offset and abnormal femoral version (Table 2.2).

• Fatigue weakness and trochanteric pain
• Hip pain when sitting (common in FAI due to impingement)
• Progression from intermittent pain to a constant dull ache
• Clicking or locking during activity

Table 2.2

Differentiating Hip Dysplasia and Femoroacetabular Impingement Hip Dysplasia Femoroacetabular Impingement Wide range of motion

Shallow, vertical acetabulum

Uncovered femoral head

Hypertropic labrum

Limited range of motion (especially internal rotation)

Normal or over-coverage

Cam or Pincer lesion on X-ray

Pistol grip femurs

When later degenerative changes occur, symptoms are more typical of hip osteoarthritis (OA), namely groin, buttock or thigh stiffness/pain related to activity levels. The differential diagnoses of extraarticular hip pathologies should be considered, particularly in the presence of normal imaging.

Clinical Examination

• Inspection - Antalgic, stiff or Trendelenburg gait (due to reduced femoral offset and abductor failure)
• Palpation - Femoral triangle pain
• Movement - Limited range of motion, crepitis, excessive internal rotation (femoral anteversion), and pain on hip flexion and internal rotation, signifying femoro-acetabular impingement

Imaging

The aims of imaging in adult hip problems are to:

1. 1. Establish the presence of dysplasia (acetabulum and femur)
2. 2. Gauge severity of dysplasia
3. 3. Identify focal pathology (e.g., FAI, labral or chondral damage)
4. 4. Evaluate presence/absence of irreversible chondral degeneration
5. 5. Plan for surgery

Plain X-rays

The typical presentation of a dysplastic hip is a shallow acetabulum. Plain X-rays are usually adequate to evaluate adult manifestations of developmental dysplasia of the hip (DDH) (Table 2.1 and Fig. 2.2). The later findings are similar to OA changes: joint space narrowing, osteophytosis and subchondral sclerosis.

Fig. 2.2 Radiological evaluation of dysplasia. a) Acetabular Index (AI), b) Centre-Edge Angle (CE), c) Crossover Sign, d) Posterior wall sign, e) Alpha angle.

Patients with missed DDH, may present in young adulthood with hip pain and leg shortening. Plain X-rays can demonstrate low and high chronic hip dislocation (Fig. 2.3).

Fig. 2.3 Hartofilakidis classification of late consequences of DDH. A) Dysplasia, B) Low dislocation, C) High Dislocation.

Computed Tomography

Computed tomography (CT) can be helpful for more detailed appreciation of coverage and version of the hip joint. It is not routinely required unless there is uncertainty regarding the anatomy and version of the acetabulum and femur.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is usually undertaken as an arthrogram with intraarticular contrast to determine if any of the following are present:

• Labral pathology
• Impingement (bone oedema, chondral changes)
• Chondral lesions
• Joint erosions

Hip Arthroscopy

Hip arthroscopy is an effective way of evaluating joint surface damage if there is an indication for a targeted therapeutic procedure, especially in borderline cases (CE angle 20-25°):

• Labral tear/detachment
• Focal chondral defect (for debridement or microfracture)

Treatment

Nonoperative

There is limited scope for nonoperative management in dysplastic hips due to the pathoanatomy and abnormal mechanics. Weight modification and gait training may give some symptomatic relief.

Operative

Hip Arthroscopy - Hip arthroscopy can be used to treat a focal lesion such as a labral detachment or partial chondral damage. It is usually only be successful in those with a borderline centre-edge angle.

Periacetabular Osteotomy - A periacetabular osteotomy (PAO) aims to increase cover of the femoral head with articular cartilage, improving the contact area in the weight-bearing zone. A range of periacetabular osteotomies have been described; however, the Ganz PAO (while technically challenging) is the predominant procedure for...