1.4 Pelvis, Femur & Patella Osteology: Landmarks & Biomechanics

1.4 Pelvis, Femur & Patella Osteology: Landmarks & Biomechanics

The Pelvic Girdle (Os Coxae): Landmarks & Foramina

The pelvic girdle is formed by the two hip bones (ossa coxae) uniting anteriorly at the pubic symphysis and articulating posteriorly with the sacrum at the sacroiliac joints. Each os coxae develops from the fusion of three primary bones—Ilium, Ischium, and Pubis—which meet at the Y-shaped triradiate cartilage within the acetabulum, fusing between ages 15 and 17.

Osseous Landmarks of the Os Coxae

1. Ilium (Superior Component):
   • Iliac Crest: Superior curvilinear border extending from ASIS to PSIS. Provides origin for the abdominal wall musculature (transversus abdominis, internal oblique, external oblique), tensor fasciae latae, and latissimus dorsi.
   • Anterior Superior Iliac Spine (ASIS): Origin of the sartorius muscle and tensor fasciae latae (TFL); anchors the lateral end of the inguinal ligament (Poupart's ligament).
   • Anterior Inferior Iliac Spine (AIIS): Origin of the straight head of the rectus femoris and the superior attachment of the massive iliofemoral ligament (Y-ligament of Bigelow). (The reflected head of the rectus femoris originates from the groove immediately superior to the acetabular rim).
   • Posterior Superior Iliac Spine (PSIS): Marked clinically by the cutaneous skin dimples of the lower back (level of S2 vertebra).
   • Greater Sciatic Notch: Deep indentation on the posterior border below the PIIS.
2. Ischium (Posteroinferior Component):
   • Ischial Spine: Sharp triangular posterior projection separating the greater sciatic notch from the lesser sciatic notch. Origin of the superior gemellus; attachment site for the sacrospinous ligament.
   • Ischial Tuberosity: Massive, rugged posteroinferior expansion that bears body weight in the seated position. Provides origin for the hamstrings (biceps femoris long head, semitendinosus, semimembranosus) and the hamstring head of the adductor magnus; anchors the stout sacrotuberous ligament.
   • Lesser Sciatic Notch: Smooth indentation inferior to the ischial spine.
3. Pubis (Anteroinferior Component):
   • Pubic Body & Symphysis: Unites with the contralateral pubic body via an amphiarthrodial fibrocartilaginous disc.
   • Pubic Tubercle: Prominent projection on the upper border of the pubic body; anchors the medial attachment of the inguinal ligament.
   • Pectineal Line (Pecten Pubis): Sharp ridge on the superior pubic ramus continuing posteriorly as the arcuate line of the ilium; provides origin for the pectineus muscle.
4. Acetabulum & Obturator Foramen:
   • Acetabulum: Deep hemispherical socket oriented laterally, inferiorly, and anteriorly. Features the lunate surface (horseshoe-shaped articular zone lined with hyaline cartilage) and the central non-articular acetabular fossa (filled with fibroelastic fat, the pulvinar). The acetabular notch is bridged by the transverse acetabular ligament, creating a foramen through which the acetabular branch of the obturator artery enters the fovea capitis.
   • Obturator Foramen: Large aperture bounded by the ischial and pubic rami, largely occluded by the fibrous obturator membrane, leaving the superior obturator canal open for the passage of the obturator nerve, artery, and vein.

Conversion of Sciatic Notches to Foramina

The greater and lesser sciatic notches are converted into closed neurovascular foramina by two dense ligaments:

• Sacrotuberous Ligament: Spans from the posterior sacrum/coccyx to the ischial tuberosity.
• Sacrospinous Ligament: Spans from the lateral sacrum/coccyx to the ischial spine.

Greater Sciatic Foramen

The piriformis muscle exits the pelvis through the greater sciatic foramen, dividing it into two distinct compartments:

1. Suprapiriform Space: Transmits the superior gluteal nerve, artery, and vein.
2. Infrapiriform Space: Transmits the:
   • Sciatic nerve (largest nerve in the human body).
   • Inferior gluteal nerve, artery, and vein.
   • Pudendal nerve and internal pudendal artery and vein.
   • Posterior femoral cutaneous nerve.
   • Nerve to obturator internus (and superior gemellus).
   • Nerve to quadratus femoris (and inferior gemellus).

Lesser Sciatic Foramen

Serves as an entry port into the perineum (ischiorectal fossa via Alcock's canal) for structures exiting the pelvis through the infrapiriform space:

• Pudendal nerve and internal pudendal vessels (course around the ischial spine/sacrospinous ligament to re-enter the pelvis).
• Tendon of the obturator internus muscle.
• Nerve to obturator internus.

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Femoral Osteology: Proximal Landmarks & Femoral Angles

The femur (thigh bone) is the longest, heaviest, and strongest bone in the human body.

Proximal Femoral Landmarks

• Femoral Head: Forms approximately two-thirds of a sphere, directed medially, superiorly, and slightly anteriorly. Features a small central depression—the fovea capitis femoris—which provides attachment for the ligamentum teres femoris and transmits a small acetabular branch of the obturator artery.
• Femoral Neck: Pyramidal strut connecting the head to the shaft. Its interior contains dense cancellous trabecular patterns (including the calcar femorale) engineered to resist intense bending stresses.
• Greater Trochanter: Large quadrangular projection on the lateral proximal femur. Sites of muscular insertion:
  • Gluteus medius: Inserts onto the lateral surface.
  • Gluteus minimus: Inserts onto the anterior surface.
  • Piriformis: Inserts onto the superior apex.
  • Obturator internus and gemelli: Insert onto the medial surface.
  • Trochanteric Fossa: Deep depression on the medial surface providing insertion for the obturator externus.
• Lesser Trochanter: Conical posteromedial projection at the junction of neck and shaft. Receives the powerful insertion of the iliopsoas tendon.
• Intertrochanteric Line (Anterior): Rough anterior ridge running from the greater to the lesser trochanter; provides attachment for the massive iliofemoral ligament.
• Intertrochanteric Crest (Posterior): Smooth posterior ridge between trochanters; bears the quadrate tubercle for insertion of the quadratus femoris.

Frontal Plane Geometry: Angle of Inclination

The angle of inclination (neck-shaft angle) is formed by the intersection of the longitudinal axis of the femoral neck and the longitudinal axis of the femoral shaft in the frontal plane:

• Normal Adult Range: 125° to 130° (infants ~150°, elderly ~120°).
• Coxa Vara (< 120°):
  • Decreased neck-shaft angle.
  • Biomechanical Effects: Increases the moment arm of the hip abductors (reducing muscle effort needed to level the pelvis), but substantially increases shear and bending forces across the femoral neck.
  • Clinical Correlate: Shortens the limb; predisposes adolescents to slipped capital femoral epiphysis (SCFE) and adults to femoral neck stress fractures.
• Coxa Valga (> 135°):
  • Increased neck-shaft angle.
  • Biomechanical Effects: Decreases the moment arm of hip abductors (requiring higher muscle force, predisposing to Trendelenburg gait) and increases compressive joint reaction forces across the superior acetabulum.
  • Clinical Correlate: Lengthens the limb; predisposes to superior hip subluxation and early osteoarthritis.

Transverse Plane Geometry: Femoral Torsion (Anteversion vs. Retroversion)

Femoral version is the angular relationship between the femoral neck axis and the transcondylar axis of the distal femoral condyles in the transverse plane:

• Normal Adult Anteversion: 12° to 15° (the femoral neck angles anteriorly relative to the condyles).
• Excessive Anteversion (> 15°–20°):
  • The femoral head rotates anteriorly. To seat the head deeply into the acetabulum during gait, the patient internally rotates the lower limb.
  • Clinical Presentation: In-toeing ("pigeon-toed") gait; physical examination reveals increased internal hip rotation (>60°) and restricted external hip rotation (<25°) on Craig's test.
• Decreased Anteversion (< 10°) or True Retroversion (negative version):
  • The femoral neck angles posteriorly relative to the condyles.
  • Clinical Presentation: Out-toeing ("duck-footed") gait; physical examination demonstrates increased external hip rotation and restricted internal hip rotation.

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Femoral Shaft & Distal Condylar Architecture

Femoral Shaft (Corpus Femoris)

• Bowed anteriorly, which enhances its structural shock-absorption capability.
• Linea Aspera: A prominent longitudinal crest on the posterior surface with medial and lateral lips:
  • Lateral Lip: Continues proximally as the gluteal tuberosity (insertion of deep gluteus maximus fibers) and distally as the lateral supracondylar line.
  • Medial Lip: Continues proximally as the spiral line and pectineal line (insertion of pectineus), and distally as the medial supracondylar line.
  • Muscular Origins/Insertions: Provides attachment for vastus lateralis, vastus medialis, vastus intermedius, adductor longus, adductor brevis, adductor magnus, and the short head of the biceps femoris.

Distal Femur & Condyles

• Medial and Lateral Condyles: Articulate with the tibial plateau and patella.
  • Lateral Condyle: Sits flatter and projects more anteriorly than the medial condyle, creating an essential anterior osseous barrier that resists lateral patellar subluxation.
  • Medial Condyle: Extends more distally than the lateral condyle, compensating for the normal femoral inclination angle to maintain a horizontal knee joint line.
• Adductor Tubercle: Distinct bony projection located atop the medial epicondyle; receives the insertion of the hamstring portion of the adductor magnus (innervated by the tibial nerve).
• Intercondylar Fossa: Deep posterior notch separating the condyles; houses the cruciate ligaments (ACL originates from medial aspect of lateral condyle; PCL originates from lateral aspect of medial condyle).

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Patellar Osteology & Articular Mechanics

Patella (Kneecap)

The patella is the largest sesamoid bone in the human body, embedded within the tendon of the quadriceps femoris.

• Biomechanical Function: Acts as an anatomical pulley, displacing the quadriceps tendon anteriorly away from the knee joint axis of rotation, increasing the quadriceps mechanical lever arm and extensor torque by 30% to 50%.
• Apex: Points distally; gives rise to the stout patellar ligament inserting into the tibial tuberosity.
• Articular Facets (Posterior Surface):
  • Lateral Facet: Larger, wider, and concave; articulates with the steeper lateral femoral trochlea.
  • Medial Facet: Smaller and slightly convex.
  • Odd Facet: A narrow vertical strip located at the extreme medial border of the medial facet. The odd facet has little or no femoral contact through most of the flexion arc; it contacts the medial femoral condyle only in deep flexion (beyond about 130°).

Bipartite Patella

• Occurs when secondary ossification centers fail to coalesce with the primary patellar center (normal patellar ossification occurs between ages 3 and 6).
• Prevalence: ~2-3% of the population, bilateral in 40-50%, predominates in males.
• Saupe Classification:
  • Type I: Inferior pole (5%).
  • Type II: Lateral margin (12%).
  • Type III: Superolateral margin (83%) (most common).
• Clinical Distinction: Bipartite patella is an incidental, asymptomatic radiographic finding characterized by smooth, rounded, sclerotic margins. Traumatic fractures exhibit sharp, irregular, non-sclerotic fracture lines.

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The Q-Angle (Quadriceps Angle) & Biomechanics

Definition & Anatomical Measurement

The Q-angle represents the angle formed by two intersecting lines in the frontal plane:

1. Line 1: Drawn from the anterior superior iliac spine (ASIS) to the center of the patella.
2. Line 2: Drawn from the center of the patella to the tibial tuberosity.

Pathomechanics of an Elevated Q-Angle (> 20°)

When the Q-angle exceeds 20°, active contraction of the quadriceps creates an abnormal lateral vector force (the "bowstring effect") that pulls the patella laterally against the lateral femoral trochlea:

• Clinical Consequences: Predisposes to patellofemoral pain syndrome (PFPS), chondromalacia patellae, lateral patellar tracking dysfunction, recurrent lateral subluxation, and frank dislocation.
• Kinetic Chain Factors Increasing Dynamic Q-Angle:
  • Excessive femoral anteversion (internally rotates the distal femur).
  • Genu valgum ("knock-knees") (displaces the knee joint medially relative to the ASIS).
  • External tibial torsion (displaces the tibial tuberosity laterally).
  • Subtalar joint overpronation (causes obligatory internal tibial rotation and dynamic valgus collapse).