Capsular attachment on the anterosuperior femoral head-neck junction: A hypothesis about femoroacetabular impingement

Abstract

Femoroacetabular impingement (FAI), characterized by a pathological contact between the proximal femur and acetabulum, is a common precursor of hip osteoarthritis. Cam morphology is a bony prominence that causes FAI and frequently forms on the anterosuperior femoral head-neck junction. Despite anatomical consensus regarding the femoral head-neck junction as a boundary area covered by the articular cartilage and joint capsule, it remains unclear whether the joint capsule is continuous with the anterosuperior articular cartilage. For the anatomical consideration of cam morphology formation, this study aimed to investigate the histological characteristics of the capsular attachment on the anterosuperior femoral head-neck junction, particularly focusing on the presence or absence of continuity of the joint capsule to the articular cartilage. A total of 21 anterosuperior regions (seven hips each for the 12:00, 1:30, and 3:00 positions) from seven hips (three males and four females; mean age at death, 68.7 years) were histologically analyzed in this study for quantitative evaluation of the capsular thickness using histological sections stained with Masson's trichrome, as well as qualitative evaluation of the capsular attachment. The present study showed that the joint capsule, which folded proximally to the femoral head-neck junction from the recess, exhibited a blend of the fibrous and synovial regions. Notably, it not only continued with the superficial layer of the articular cartilage, but also attached to the articular cartilage via the fibrocartilage. This continuous region was relatively fibrous with dense connective tissue running in the longitudinal direction. The capsular thickness at the recess point (mean, 1.7 ± 0.9 mm) and those at the distal end of the articular cartilage (0.35 ± 0.16 mm) were significantly greater than the control value for the most superficial layer thickness of the articular cartilage (0.019 ± 0.003 mm) (Dunnett's T3, both p-value <0.001). Based on the fibrous continuity between the joint capsule and articular cartilage and its thickness, this study suggests the anatomical possibility that some mechanical stress can be transmitted from the joint capsule to the articular cartilage at the frequent sites of cam morphology.

Keywords: articular cartilage; cam morphology; femoral head–neck junction; hip joint capsule; histology.

FIGURE 1

Radial plane locations on the micro‐CT images of the proximal femur. (a) Proximal aspect perpendicular to the femoral neck axis. The radial planes rotate clockwise in 45° intervals around the femoral neck axis (green dot). The 12 o'clock plane is parallel to the axis of the proximal femur diaphysis. (b) Anteromedial aspect of the proximal femur. The radial boxed regions are analyzed, as shown in Figure 2a (12:00), 3a (1:30) and 4a (3:00). Ant, Anterior; CT, computed tomography; Green line, femoral neck axis; Lat, lateral; Sup, superior.

FIGURE 2

Capsular attachment on femoral head–neck junction at the 12:00 position. (a) Micro‐CT images of the boxed region (12:00) in Figure 1b. (b) Macroscopic section of the white boxed region in Figure 2a. (c) Histological section (Masson's trichrome staining) of the dotted boxed region in Figure 2b. Femoral capsular attachment makes the recess (T1, capsular thickness at the recess point) and partly extends to the femoral head–neck junction. Black arrow heads indicate the fibrous region and red open arrow heads indicate the synovial region. (d) Magnified images of the black boxed region in Figure 2c. Joint capsule (Cap) not only continues to the superficial layer of the articular cartilage (AC) but also attaches to the AC via the fibrocartilage (asterisk). T2 indicates the capsular thickness at the distal end of the AC. (e) and (f) Correspond Safranin O staining sections with (c) and (d), respectively. CT, computed tomography; GMa, gluteus maximus; GMe, gluteus medius; GMi, Gluteus minimus; GT, Greater trochanter; HF, head of femur; Labrum, acetabular labrum; NF, neck of femur; OI, Obturator internus; RFr, reflected head of the rectus femoris.

FIGURE 3

Capsular attachment on femoral head–neck junction at the 1:30 position. (a) Micro‐CT images of the boxed region (1:30) in Figure 1b. (b) Macroscopic section of the white boxed region in Figure 3a. (c) Histological section (Masson's trichrome staining) of the dotted boxed region in Figure 3b. T1 indicates capsular thickness at the recess point. Black arrow heads indicate the fibrous region and red open arrow heads indicate the synovial region. (d) Magnified images of the black boxed region in Figure 3c. Joint capsule (Cap) partly attaches to the articular cartilage (AC) via the fibrocartilage (asterisk). T2 indicates the capsular thickness at the distal end of the AC. AIIS, anterior inferior iliac spine; CT, computed tomography; GMe, gluteus medius; GMi, Gluteus minimus; GT, Greater trochanter; HF, head of femur; Ip, iliopsoas; Labrum, acetabular labrum; NF, neck of femur; RF, rectus femoris; Sa, sartorius; TFL, tensor fascia latae.

FIGURE 4

Capsular attachment on femoral head–neck junction at the 3:00 position. (a) Micro‐CT images of the boxed region (3:00) in Figure 1b. (b) Macroscopic section of the white boxed region in Figure 4a. (c) Histological section (Masson's trichrome staining) of the dotted boxed region in Figure 4b. T1 indicates capsular thickness at the recess point. Black arrow heads indicate the fibrous region and red open arrow heads indicate the synovial region. (d) Magnified images of the black boxed region in Figure 4c. Joint capsule (Cap) partly attaches to the articular cartilage (AC) via the fibrocartilage (asterisk). T2 indicates the capsular thickness at the distal end of the AC. CT, computed tomography; GMi, gluteus minimus; GT, Greater trochanter; HF, head of femur; Ip, iliopsoas; Labrum, acetabular labrum; NF, neck of femur; RF, rectus femoris; Sa, sartorius; TFL, tensor fasciae latae; VL, vastus lateralis.

FIGURE 5

Schematic diagram of the capsular attachment on femoral head–neck junction. Comparison of the conventional understanding (a) and present findings (b) at the 12 o'clock position. The joint capsule (Cap) has generally been reported to end at the distal margin of the articular cartilage (AC) on the femoral head (HF); however, the present study revealed that the anterosuperior cap continued to the superficial layer of the AC and attached to the AC via the fibrocartilage (asterisk). GMi, gluteus minimus; Labrum, acetabular labrum; NF, neck of femur; RFr, reflected head of rectus femoris.