JBJS | Article

The Journal of Bone & Joint Surgery, Volume 80, Issue 12

Articles | December 01, 1998

The Role of the Acetabular Labrum and the Transverse Acetabular Ligament in Load Transmission in the Hip*

GREGORY A. KONRATH, M.D.†; ANDREW J. HAMEL, B.B.S.‡; STEVE A. OLSON, M.D.§; BRIAN BAY, PH.D.§; NEIL A. SHARKEY, PH.D.‡, SACRAMENTO, CALIFORNIA

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Investigation performed at the Department of Orthopaedic Surgery, University of California-Davis Medical Center, Sacramento

The Journal of Bone & Joint Surgery. 1998; 80:1781-8

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Abstract

We performed a biomechanical study of seventeen hip joints in the pelves of nine cadavera in order to assess the role that the acetabular labrum and the transverse acetabular ligament play in load transmission. The distribution of contact area and pressure between the acetabulum and the femoral head was measured with the hip in four different conditions: intact (seventeen hips), after removal of the transverse acetabular ligament (eight hips), after removal of the entire labrum (nine hips), and after removal of both the transverse acetabular ligament and the labrum (seventeen hips). The hip joint was loaded in simulated single-limb stance, and the measurements were made with use of pressure-sensitive film.A peripheral distribution of load was seen in the intact acetabula. This pattern was altered only minimally after removal of the transverse acetabular ligament or the labrum, or both. When both of these structures were removed, the only significant change was a decrease in the maximum pressure in the posterior aspect of the acetabulum (p = 0.02). No significant changes were detected with regard to the contact area, load, mean pressure, or maximum pressure in the anterior or superior aspect of the acetabulum under any testing condition.CLINICAL RELEVANCE: Our findings indicate that removal of the transverse acetabular ligament or the labrum, or both, does not significantly increase pressure or load in the acetabulum and may not predispose the hip to premature osteoarthrosis.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Tears of the acetabular labrum have received increased attention in the orthopaedic literature as a cause of hip pain. The operative treatment of labral tears consists primarily of partial or total labral resection (labrectomy) by means of either arthrotomy or arthroscopy. The short-term results have been good after this treatment^16,25, but the long-term effects are not yet known. The paucity of biomechanical data and long-term clinical studies has left orthopaedic surgeons with few guidelines. A similar scenario was encountered in the treatment of meniscal tears of the knee earlier in this century. Before long-term clinical studies of the results of total meniscectomy were available, complete resection of the meniscus for the treatment of symptomatic tears was done frequently, with excellent short-term outcomes; however, premature osteoarthrosis eventually developed in many of these patients^3,15,41. The role of the meniscus in load distribution and shock absorption in the knee subsequently was documented in several biomechanical studies^{2,17,27,31,38}.

The biomechanics of the hip joint have been evaluated extensively with use of in vitro testing^4,8,14,23,34, mathematical models^{7,19,21,30,36}, and instrumented endoprostheses^6,24,37. In contrast to the meniscus, however, the fibrocartilaginous acetabular labrum and, specifically, its role in load transmission in the hip joint have not been studied, to our knowledge. If the labrum helps to distribute loads and pressures in the acetabulum, then resection may result in increased articular pressures and lead to premature osteoarthrosis²¹. We undertook the current biomechanical study to determine if the labrum participates in load transmission and whether resection alters force, pressure, contact area, or a combination of these parameters in the hip joint.

The function of the transverse acetabular ligament is less clinically relevant. Isolated damage to or resection of this ligament has not been reported in the orthopaedic literature, to our knowledge, although disruption of the ligament may be assumed to occur in such injuries as displaced acetabular fractures. Nonetheless, we tried to determine if the transverse acetabular ligament performs as a tension band, resisting anteroposterior widening during loading of the hip joint.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†Lafayette Orthopedic Clinic, 2525 South Street, Lafayette, Indiana 47904-3075.

‡Pennsylvania State University, University Park, Pennsylvania 16802.

§Department of Orthopaedic Surgery, University of California-Davis Medical Center, 4860 Y Street, Suite 3800, Sacramento, California 95817. Please address requests for reprints to Dr. Olson.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†Lafayette Orthopedic Clinic, 2525 South Street, Lafayette, Indiana 47904-3075.

‡Pennsylvania State University, University Park, Pennsylvania 16802.

§Department of Orthopaedic Surgery, University of California-Davis Medical Center, 4860 Y Street, Suite 3800, Sacramento, California 95817. Please address requests for reprints to Dr. Olson.

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Fig. 1 Drawing showing the model that simulated single-limb stance. W = body weight, A = abductor mechanism, and J = joint-reaction force.

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Fig. 2 Drawing showing a lateral view of the acetabulum with the labrum and the transverse acetabular ligament intact.

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Fig. 3 Fuji film patterns of a hip from Group 2. Note the peripheral pattern of loading in the intact specimen. This pattern was grossly unchanged after removal of the transverse acetabular ligament (TAL) or the labrum, or both.

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Figs. 4, 5, and 6: Graphs showing the contact area, mean pressure, maximum pressure, and load in the study groups. An asterisk indicates that the difference between the values was significant (p = 0.05). Fig. 4: Group 1: nine hips from which the labrum had been removed.

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Fig. 5 Group 2: eight hips from which the transverse acetabular ligament (TAL) had been removed.

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Fig. 6 All seventeen hips, after both the labrum and the transverse acetabular ligament (TAL) had been removed.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Nine whole pelves with eighteen articulated proximal femoral segments were obtained from fresh human cadavera. The specimens were cleaned of soft tissues, leaving the pelvic ligaments and capsules of both hip joints intact. One hip joint had evidence of advanced osteoarthrosis and therefore was excluded. The remaining seventeen specimens had no radiographic or macroscopic evidence of disease. The specimens were stored at -20 degrees Celsius until the time of testing.

The pelvis was prepared and was mounted onto a load-cell in the crosshead of a materials testing machine (Instron, Canton, Massachusetts) in a manner that has been described previously^5,34 (Fig. 1). The plane formed by the anterior superior iliac spines and the pubic symphysis was aligned vertically. The ischial tuberosities were aligned so that a line drawn between them in the coronal plane was parallel to the floor. The potted femoral shaft was parallel to the coronal plane, in 15 degrees of adduction in the coronal plane relative to a reference line perpendicular to the line drawn between the ischial tuberosities in the coronal plane, and in 5 to 10 degrees of internal rotation. The relative position of the pelvis and the femur was selected on the basis of the study by McLeish and Charnley³⁰. Rotation was measured relative to the femur being in neutral rotation when the linea aspera was directly posterior. The orientation of the joint-reaction force vector was parallel to the coronal plane and angled 25 degrees superior and medial relative to a reference line perpendicular to the line drawn between the ischial tuberosities in the coronal plane^6,30.

The abductor mechanism was simulated with use of a continuous cable that was threaded through pulleys, the attachments of which were distributed along the iliac crest and at the level of the greater trochanter¹¹. The cable was connected in series to a force transducer and a computer-controlled linear actuator that was mounted to the femoral pot. This construct was designed to reproduce the normal force vector of the abductor mechanism⁹. The abductor mechanism then was used to load the hip joint while real-time measurements of abductor force and crosshead force were recorded. The crosshead of the materials testing machine measured but did not actively input external load. The hip joint was loaded until a force that was equal to the body weight of the cadaver (as detected by the crosshead load-cell) was achieved at the sacrum; the load then was maintained for thirty-five seconds. The magnitude and direction of the axial and abductor loads were used in order to calculate the magnitude and direction of the joint-reaction force across the hip.

Articular contact pressures were recorded with use of low-range (2.5 to ten-megapascal) and, as needed, medium-range (ten to fifty-megapascal) Fuji prescale film (Itochu International, New York, N.Y.). The moistened femoral head was covered with a thin layer of latex (a Trojan latex condom; Carter Wallace, New York, N.Y.). Two layers of prescale film were cut into a star shape and were applied to the latex with photographic mounting adhesive. A second layer of latex then was placed over the film. The latex-film-latex construct was 250 micrometers thick.

The joint capsule was removed before testing. The specimens were kept moist with normal saline solution throughout testing. Two holes, each with a diameter of two millimeters, were drilled through the acetabular articular surface at points 30 degrees on each side of the acetabular vertex, midway between the cotyloid fossa and the acetabular rim. Blunt probes were inserted through these holes under full load to mark the pressure-sensitive film on the femoral head for orientation during analysis. The acetabular rim also was outlined on the film while the joint was loaded. The joint was loaded twice in each condition. Room temperature and humidity were recorded for each test.

Testing first was performed with the acetabulum intact (Fig. 2). The hips then were randomized into one of two groups. In Group 1, the entire labrum was resected and the hip was retested. The transverse acetabular ligament then was removed, and the hip was reloaded. In Group 2, the transverse acetabular ligament was removed and the hip was tested. The labrum then was removed, and the hip was retested.

After loading, the pressure-sensitive film was removed from the femoral head, aligned on a sheet of white paper, and laminated in plastic to facilitate handling. All films were analyzed with use of a three-stage process of digitizing, filtering, and measurement. A digitized image of the prescale film was made with a flatbed scanner (Abaton Scan 300/GS; Everex Systems, Fremont, California). This image then was filtered and was measured with Image software (Wayne Rasband, Research Services Branch, National Institutes of Health, Bethesda, Maryland) on a Power Macintosh 7100 computer (Apple, Cupertino, California).

Calibration curves were produced for both the low and the medium-range film with use of a small Delrin cylinder (DuPont, Wilmington, Delaware) mounted in the materials testing machine to generate known pressures in latex-film-latex constructs. The calibration curve related film density to pressure. This curve was in close agreement with the low-humidity curve supplied by the manufacturer; however, the former curve reflected testing conditions in our laboratory more accurately and thus was used for all measurements.

On the basis of reference marks that were established during testing, the images were divided into three regions: the anterior and posterior walls of the acetabulum and the superior aspect of the acetabulum. This division separated the acetabulum approximately into thirds. The contact area with a pressure of 2.5 megapascals or more also was measured. The contact force in each region was calculated as the product of the mean pressure and the area. Data from duplicate runs for each hip under each condition were averaged and were analyzed with use of repeated-measures pairwise comparisons. Significance was set at the 95 percent confidence level. The joint-reaction forces were calculated from the film imprints by extracting the superiorly directed component of the total joint-contact force. The spherical shape of the joint was accounted for in the calculations. The joint-reaction forces in the hips under the four testing conditions were compared with use of repeated-measures analysis of variance with Bonferroni adjustments. The joint-reaction forces that were calculated from the pressure patterns were compared with applied loads and were analyzed with a paired t test to verify the fidelity of load recovery.

Results

Introduction | Materials and Methods | Results | Discussion | References

Seventeen hips in the pelves of nine cadavera were tested. The cadavera were those of five women and four men whose ages had ranged from fifty-two to eighty-three years (mean, seventy-two years) at the time of death. Seventeen hips were tested intact; eight, with only the transverse acetabular ligament removed; nine, with only the labrum removed; and all seventeen, with both the transverse acetabular ligament and the labrum removed.

The joint-reaction forces, as calculated from the pressure patterns, were a mean (and standard deviation) of 2350 ± 1460 newtons in the intact hips, 2340 ± 875 newtons in those from which the transverse acetabular ligament had been removed, 2210 ± 1075 newtons in those from which the labrum had been removed, and 2180 ± 955 newtons in those from which both the transverse acetabular ligament and the labrum had been removed. No significant differences were detected among these groups (p = 0.32). The mean joint-reaction force, as calculated from the pressure patterns under all testing conditions, was 2230 ± 1195 newtons. The mean applied load was 2060 ± 890 newtons. No significant difference was detected when we compared the mean applied load versus the mean joint-reaction force for all of the hips tested (p = 0.12).

A peripheral pattern of loading was seen in the anterior, superior, and posterior regions of the intact acetabula. This pattern was not grossly changed after removal of the transverse acetabular ligament or the labrum, or both, from any of the specimens (Fig. 3).

Group 1: Removal of the Labrum (Nine Hips)

After removal of the labrum, no significant changes were detected in the contact area (p = 0.90), mean pressure (p = 0.24), maximum pressure (p = 0.25), or load (p = 0.28) in the anterior aspect of the acetabulum (Fig. 4). Similarly, no significant changes were detected in the contact area (p = 0.34), mean pressure (p = 0.33), maximum pressure (p = 0.98), or load (p = 0.66) in the superior aspect of the acetabulum. There was a significant decrease in the maximum pressure (p = 0.02) and the mean pressure (p = 0.05) in the posterior aspect of the acetabulum, but no significant change in the contact area (p = 0.25) or the load (p = 0.20) was detected in that region.

Group 2: Removal of the Transverse Acetabular Ligament (Eight Hips)

After removal of the transverse acetabular ligament, no significant changes were detected in the contact area (p = 0.82), mean pressure (p = 0.39), maximum pressure (p = 0.43), or load (p = 0.51) in the anterior aspect of the acetabulum (Fig. 5). Similarly, no significant changes were detected in the contact area (p = 0.28), mean pressure (p = 0.85), maximum pressure (p = 0.56), or load (p = 0.43) in the superior aspect of the acetabulum. There was a significant decrease in the load (p = 0.03) and the contact area (p = 0.05) in the posterior aspect of the acetabulum, but no significant change in the mean pressure (p = 0.95) or the maximum pressure (p = 0.68) was detected.

Groups 1 and 2: Removal of the Transverse Acetabular Ligament and the Labrum (Seventeen Hips)

After removal of both the transverse acetabular ligament and the labrum, no significant changes were detected in the contact area (p = 0.77), mean pressure (p = 0.33), maximum pressure (p = 0.38), or load (p = 0.36) in the anterior aspect of the acetabulum (Fig. 6). Similarly, no significant changes were detected in the contact area (p = 0.99), mean pressure (p = 0.41), maximum pressure (p = 0.90), or load (p = 0.39) in the superior aspect of the acetabulum. There continued to be a significant decrease in the maximum pressure (p = 0.02) in the posterior aspect of the acetabulum, but it was no longer possible to detect significant changes in the contact area (p = 0.39), mean pressure (p = 0.08), or load (p = 0.19). The statistical power of the comparisons was 0.85 in detecting differences of 0.6 megapascal in the mean pressure and 1.5 megapascals in the maximum pressure in the superior aspect of the acetabulum between the intact hips and those from which the transverse acetabular ligament and the labrum had been removed.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

The transverse acetabular ligament is a fibrous tissue link spanning the inferior acetabular notch. This ligament connects the anteroinferior and posteroinferior horns of the semilunar surface of the acetabulum. The posterior aspect of the ligament attaches to the bone beneath the lunate surface, and the anterior aspect attaches to the labrum³¹. The transverse acetabular ligament contains no cartilage cells¹⁸. The function of this ligament in the hip is currently unknown.

Lohe et al. reported a 3.2 percent increase in the length of the transverse acetabular ligament when the hips of cadavera were subjected to loads of 2800 newtons²⁸. A maximum strain of 2.5 percent was calculated as occurring during the flat-foot phase of the gait cycle, the position that was tested in simulated single-limb stance in the current study. This testing confirmed that anteroposterior widening of the acetabulum occurs during loading, but it did not indicate whether the transverse acetabular ligament is a major antagonist to widening.

We and other authors²² have conjectured that the transverse acetabular ligament might function as a tension band between the posteroinferior and anteroinferior aspects of the acetabulum, resisting anteroposterior widening of the acetabulum during loading of the joint. The concept that we have found to be most useful and that is supported by substantial experimental data is that the hip functions as an incongruous joint, with the unloaded acetabulum having a smaller diameter than the femoral head^1,9,19,20. The acetabulum deforms about the femoral head when it is loaded. The acetabulum undergoes elastic deformation to become congruous with the femoral head, and contact is made about the periphery of the anterior, superior, and posterior articular surfaces of the acetabulum. This mechanism is vital to the optimization of load distribution in the hip and may contribute to the nutrition of articular cartilage⁹.

This concept implies that the elastic behavior of the pelvic ring and the action of the abductor muscles are both important determinants of acetabular and pelvic deformation. Finite element analysis¹³ has shown that substantial strains occur at the pubic symphysis and the sacroiliac joint during loading of the hip, indicating the importance of these structures in supporting hip-joint loads. The technical drawbacks of our method of testing, which included the use of older donors, the inability of the film to measure pressures of less than 2.5 megapascals, the thickness artifact (250 micrometers) of the latex-film-latex construct, and the long loading interval (which may allow creep deformation of cartilage⁴), were largely overcome by the repeated-measures design of our study. Our experimental preparation simulated static single-limb stance; we do not know if this is the most critical position for observing the consequences of an acetabular fracture, but it is a condition in which the hip sustains large loads and it has been used widely in other studies^{6,12,21,29,30}.

In a previous study, we demonstrated that removing the acetabulum from the pelvis, fixing it in polymethylmethacrylate, and loading the hip joint by axial compression of the femur, as has been done frequently in other investigations^1,14,32, destroyed the normal load-transmitting qualities of the joint⁵. Therefore, we chose a testing method that preserves the entire pelvis and loads the joint by simulated action of the abductor muscles. Previous experiments in our laboratory consistently revealed peripheral loading in the hip in simulated single-limb stance when the pelvis was intact^5,22,34,35. Major alterations in the structural integrity of the acetabulum—when it is removed from the pelvis⁵, after a simulated acetabular fracture^22,34,35, or (potentially) when certain soft tissues are removed—can cause a decrease in the contact area and pressures in the anterior and posterior aspects of the acetabulum, with concomitant increases in the superior aspect. Increased articular contact pressures in the superior aspect, as seen in association with dysplasia of the hip²¹ and certain displaced acetabular fractures^22,34,35, have been found to be related to premature degenerative changes in the hip.

Decreases in the contact area and the load in the posterior region were the only significant changes seen in the acetabulum after removal of the transverse acetabular ligament. These decreases were consistent with some of the alterations seen after a simulated acetabular fracture^22,34,35; however, we could no longer detect a significant difference when the labrum also was removed and more specimens were assessed. There also was a lack of concurrent changes in the anterior and superior aspects of the acetabulum, which indicates that the initial findings in the posterior aspect probably were not consequential, statistical significance notwithstanding. The marked increases in articular pressure in the superior aspect of the acetabulum that are associated with premature degeneration of the joint were notably absent. This finding indicates that the transverse acetabular ligament is not a principal antagonist to anteroposterior widening during loading in single-limb stance.

The acetabular labrum is a fibrocartilaginous rim that is attached to the acetabular margin and deepens the acetabular cup. It is triangular in section and is attached at its base to the acetabular rim; its apex is its free margin¹⁸. Histological examination of the labrum has shown free nerve-endings and sensory end organs in its superficial layers²⁶. These nerve-endings may participate in nociceptive and proprioceptive mechanisms. The acetabular labrum deepens the hip socket in a fashion that is similar to the way that the glenoid labrum deepens the shoulder socket. However, in contrast to the glenoid in the glenohumeral joint, the osseous acetabulum in the hip is much deeper and provides substantial static stability to the hip joint. The deepening of the acetabulum that is provided by the labrum is therefore less important. Some research does indicate, however, that the labrum may enhance stability by providing negative intra-articular pressure in the hip joint^40,42. Any capacity that the labrum has to share loads with the acetabular articular cartilage or to resist anteroposterior widening of the acetabulum during loading has not been evaluated, to our knowledge.

The clinical importance of any usefulness of the labrum in load transmission in the hip is relevant in light of the increasing attention that the treatment of labral tears is receiving in orthopaedics. These symptomatic tears can occur posterosuperiorly or anteriorly after relatively minor trauma^16,25. The diagnosis can be made on the basis of physical examination, arthrography, magnetic resonance imaging with intravenous or intra-articular administration of contrast medium^12,33, or arthroscopy^10,29,39. Nonoperative treatment includes protected weight-bearing and use of nonsteroidal anti-inflammatory medication. Operative treatment consists of arthrotomy or arthroscopy with resection of the entire labrum or the portion of the labrum that is torn. Several investigators have reported excellent and good short-term results after resection of labral tears^16,25. The long-term outcomes have not yet been reported, to our knowledge. We sought to evaluate load transmission in the hip after labrectomy to determine whether there were any changes that could predispose the hip joint to early osteoarthrosis. As mentioned previously, any increases in articular pressures were specifically looked for.

In contrast to the meniscus in the knee, the acetabular labrum does not increase contact area, distribute load, or reduce contact stresses in the hip. The alterations in load transmission following labrectomy were limited to the posterior aspect of the acetabulum, in which the mean and maximum pressures were significantly decreased. The significant decrease in maximum pressure remained when the transverse acetabular ligament also was removed and more specimens were evaluated. No significant changes were detected in the anterior or superior aspect of the acetabulum. There also were no increases in the pressure or load and no alterations in the contact area in any region following labrectomy.

It is clinically important that no increases in pressures were noted. The statistical power of comparisons of 0.85 in detecting differences of 0.6 megapascal in the mean pressure and 1.5 megapascals in the maximum pressure in the superior aspect of the acetabulum indicates that any meaningful increases in this region after removal of the labrum and the transverse acetabular ligament would have been detected. Significant increases of 0.63 megapascal in the mean pressure (p = 0.01) and 1.62 megapascals in the maximum pressure (p = 0.01) were noted in the superior aspect of the acetabulum when intact acetabula were compared with those that had a simulated fracture of the posterior wall in a previous study in our laboratory³⁵.

The potential contribution of the transverse acetabular ligament or the labrum to stabilization, proprioception, shock absorption, or lubrication of the hip joint is still unknown. The findings in our study of a cadaver model should therefore be interpreted with caution when applied to a clinical setting. Nonetheless, these preliminary data suggest that labrectomy or removal of the transverse acetabular ligament, or both, does not significantly increase articular pressures in the hip joint during single-limb stance and may not predispose the hip to premature osteoarthrosis.

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Introduction | Materials and Methods | Results | Discussion | References

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