The labrum: structure, function, and injury with femoro-acetabular impingement

Abstract

Background: The recognition of the importance of femoro-acetabular impingement (FAI) as a potential cause of hip pain has been stimulated by major efforts to salvage hip joints by reconstruction to prevent or delay the need for replacement. A previous review addressed the nature of FAI, the various types, and how to make the diagnosis. When FAI occurs, the structure between the femur and acetabular rim, the labrum, is initially impinged upon and subsequently injured.

Method: Injury to the labrum should be recognized when treating the osseous causes of FAI. Preserving or recovering labral function, enhancing hip stability and protecting the articular surface, is critical to restoring the hip to normal or near-normal mechanical and physiologic function. The present review collected the varied essential information about the labrum in a succinct manner, independent of treatment algorithms.

Results/conclusion: Advanced knowledge of the labrum is presented, including the anatomy, circulation, histology, embryology, and neurology, as well as how the labrum tears, the types of tears, and how to make the diagnosis. The advantages and limitations of diagnostic magnetic resonance techniques are discussed, including magnetic resonance imaging (MRI), indirect magnetic resonance arthrography (i-MRA), and direct magnetic resonance arthrography (d-MRA). The review recognizes the complexity of the labrum and provides a greater understanding of how the labrum is capable of stabilizing the joint and protecting the articular surface of the hip. This information will act as a guide in developing treatment plans when treating FAI.

Keywords: Consolidation; Labrum; MRA; MRI; Magnetic resonance arthrography; Magnetic resonance imaging.

Fig. 1

The labrum and transverse ligament are a continuous structure (modified with permission from Grant [3], Fig. 261)

Fig. 2

The pubocapsular, iliofemoral, and ischiofemoral ligaments. The ligament teres femoris arises from the pillars to the acetabular notch and the transverse acetabular ligament (modified with permission from Gray [4]: (a) p. 312, Fig. 336; (b) p. 313, Fig. 337; (c) p. 314, Fig. 338)

Fig. 3

The structure of the labrum. Note the capsular recess (modified with permission from Seldes et al. [5], Fig. 1)

Fig. 4

Collagen fiber attachment of the anterior and posterior labrum (modified with permission from Cashin et al. [7], Figs. 2ab and 3ab)

Fig. 5

Embryology of the labral acetabular complex. Fetal hip at term. (A) acetabulum, (B) femur, (C) anterior labrum, (D) intra-articular projection, (E) posterior labrum (note that there is no intra-articular projection), and (F) acetabular labrum transition zone (modified with permission from Cashin et al. [7], Fig. 1)

Fig. 6

The (1) superior and (2) inferior gluteal arteries, the (3) medial and (4) lateral circumflex arteries contribute to the retinaculum (5), which supplies the labrum (reproduced with permission from Kalhor et al. [9], Fig. 8)

Fig. 7

The labrum and capsule are supplied primarily by the superior and inferior gluteal arteries. The terminal branches of the capsular retinaculum are clearly demonstrated (modified with permission from Kalhor et al. [9], Fig. 6)

Fig. 8

The labrum zones of circulation: a Zone II—inner or articular side and c Zone I—outer or capsular side. Each of these is then divided into the peripheral part, IA and IIA, and the part closest to the acetabular rim, IB and IIB. The main vascular source is from the bony rim that primarily supplies Zone IB, the capsular side of the labrum closest to the rim (green arrow and open arrow). From there, the circulation extends to the periphery, to the peripheral capsular Zone IA (red arrow and white arrow). Through this peripheral capsular side, Zone IA, the peripheral articular side, Zone IIA is supplied. The inner articular side, merging with the articular cartilage, Zone IIB is relatively avascular (black arrow) (modified with permission from Kelly et al. [11]: (a) p. 9, Fig. 6; (b) p. 6, Fig. 3A; (c) p. 10, Fig. 7)

Fig. 9

Hypertrophic femoral neck changes from abnormal acetabular edge contact (modified with permission from Allen et al. [15], Fig. 3)

Fig. 10

Three-dimensional representation of the abnormal labral acetabular edge contact that occurs anteriorly and at the posterior cartilage–labrum surface with slipped capital femoral epiphysis (SCFE) (modified with permission from Richolt et al. [16], Figs. 3 and 4)

Fig. 11

The labrum, stiff under tension, acts as a seal. The joint space, thus, contains a 0.4-mm layer of pressurized fluid, which protects the articular surfaces. Contact is resisted by the high fluid pressures and the tight labral ring (black arrow), where the peak strain can reach 2 %. This pressurizes the interstitial fluid within the cartilage. This pressurization resists “consolidation” or the forcing of fluid out of the cartilage (reproduced with permission from Ferguson et al. [18], Fig. 1)

Fig. 12

Types of tears of the labrum. a Pincer type femoro-acetabular impingement—tears within the labrum. b Cam type impingement—cleavage separation between the labrum and articular cartilage (modified with permission from Tannast and Siebenrock [20], Fig. 2)

Fig. 13

Tears of the labrum with cam and pincer impingement (modified with permission from Seldes et al. [5], Figs. 5 and 6)

Fig. 14

a Labral tears from pincer femoro-acetabular impingement (FAI). b Labrum articular cartilage cleavage with delamination of articular cartilage from cam FAI (modified with permission from Ganz et al. [21], Fig. 5a, b)

Fig. 15

(I) No surface labral damage, (II) labral surface damage, (III) labral articular cartilage separation, (A) no intrasubstance labral cysts, (B) intrasubstance labral cysts (modified with permission from Czerny et al. [22], Fig. 2)

Fig. 16

Most anterior impingement that causes labral tears occurs mainly in zone 1, but also in zones 2 and 3. Posterior tears are seen in zones 4 and 5 (modified with permission from Ilizaliturri et al. [14], Fig. 3)

Fig. 17

Lateral and radial Images of femoral head–neck offset. The asterisk shows the area of acetabulum/labrum abnormal zone (white area) (reproduced with permission from Hack et al. [25], Fig. 3C)

Fig. 18

a Normal head–neck offset (OS). b Abnormal head–neck offset (OS’) due to hypertrophic femoral neck changes (reproduced with permission from Tannast and Siebenrock: [20], Fig. 14)

Fig. 19

Normal alpha angle among men was 68° or less and in women, it was 50° or less (reproduced with permission from Allen et al. [15], Fig. 1)

Fig. 20

a Method of taking X-rays to measure the beta angle. b The beta angle defines limits for normal (average 38.7°) and FAI (average 15.6°) (modified with permission from Brunner et al. [29]: p. 1204, Fig. 1; p. 1205, Fig. 2)

Fig. 21

Indirect magnetic resonance arthrography of the hip at 3T with radial images. Top left: en face image of the acetabular opening with overlaid radial sections. Top right: plane of section for the bottom left image. Bottom left: fat suppressed, proton density radial image showing tear of the anterosuperior labrum (modified with permission from Petchprapa [32])