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Relationship Between Offset Angle Alpha and Hip Chondral Injury in Femoroacetabular Impingement
Relationship Between Offset Angle Alpha and Hip Chondral Injury in Femoroacetabular Impingement Todd L. Johnston, M.D., Mara L. Schenker, B.S., Karen K. Briggs, M.P.H., and Marc J. Philippon, M.D.
Purpose: The purpose of this study was to examine the relationship between the size of cam lesions and the presence of cartilage damage, labral damage, or changes in range of motion in the hips with signs and symptoms of femoroacetabular impingement (FAI). Methods: Cross-table lateral radiographs were available for 102 consecutive patients presenting with signs and symptoms of FAI. Radiographs with excessive external rotation, dysplasia, severe arthritis, avascular necrosis, or Legg-Calvé-Perthes syndrome were excluded, leaving 82 patients available for analysis (47 men, 35 women; average age, 25 yr [range, 12 to 55 yr]). Offset angle alpha was measured from the films with a digital goniometer. Patients subsequently underwent hip arthroscopy and the surgical findings and hip range of motion were prospectively recorded. Results: Higher offset angle alpha was associated with the presence of acetabular rim chondral defects (P ⫽ .044) and full-thickness delamination of the acetabular cartilage (P ⫽ .034). Patients with detachment of the base of the labrum had a higher offset angle alpha (P ⫽ .016). Higher offset angle alpha was related to male sex (P ⫽ .001) and decreased range of motion (P ⬍ .05), but not to age. Conclusions: Cam-type FAI, as measured by an increased offset angle alpha, was correlated with increased chondral damage, labral injury, and decreased range of motion. Level of Evidence: Level II, development of diagnostic criteria on basis of consecutive patients with universally applied gold standard. Key Words: Alpha angle—Cam— Femoroacetabular impingement—Hip arthroscopy—Offset angle alpha.
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ust under 30% of published cases of hip osteoarthritis are attributed to primary osteoarthritis in which the cause is unknown.1 Significant recent advances in our understanding of hip pathology have led to the hypothesis that small alterations in hip morphology can cause motion-induced mechanical damage over time.2-4 The term femoroacetabular impingement (FAI) has been coined to describe these morphologic
From Cedar Valley Medical Associates (T.L.J.), Waterloo, Iowa, and Steadman Hawkins Research Foundation (M.L.S., K.K.B., M.J.P.), Vail, Colorado, U.S.A. Supported in part by a grant from Smith & Nephew. Address correspondence and reprint requests to Marc J. Philippon, M.D., Steadman Hawkins Research Foundation, Attn: Clinical Research, 181 W. Meadow Dr., Ste. 1000, Vail, CO 81657, U.S.A. E-mail: [email protected] © 2008 by the Arthroscopy Association of North America 0749-8063/08/2406-7302$34.00/0 doi:10.1016/j.arthro.2008.01.010
alterations, which have been implicated as a potential cause of primary hip osteoarthritis.2 There are two described subtypes of hip impingement: cam and pincer.5 Cam impingement results from excessive bone at the femoral head–neck junction that extends beyond the normal sphericity of the femoral head, or when there is a flattening and loss of the normal offset between the femoral head and neck.5 Pincer impingement results from excessive bone along the acetabular rim or relative acetabular retroversion.4 More than 80% of patients presenting with FAI are reported to have a combination of the two subtypes.6 The etiologies of these morphologic lesions are not well understood. Observations made during open surgical treatment for FAI have led to the hypothesis that FAI may be a cause of early osteoarthritis.2,6 Surgical findings suggest that cam impingement, in particular, can lead to significant chondral and labral injuries when the areas of excess bone on the proximal femoral neck are
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forced into the joint with motion. This incongruence is thought to lead to increased friction and a concentration of shear forces parallel to the acetabular rim cartilage and labral structures. Ganz et al.2 have suggested that as the area of femoral incongruence enters into the acetabulum, the labrum is pushed up and the shear stresses are concentrated on the labral– chondral junction, leading to a separation. The extent of the chondral damage has been observed to vary from small areas of chondral delamination to larger, fullthickness lesions as the base of the cartilage is sheared and pulled up like the edge of a carpet. Labral detachment is also a frequent finding and may occur at the base of the labrum near the labral– chondral junction. Radiographic methods for defining cam impingement have advanced in the last few years. One instrument for quantifying the extent of a cam lesion is the offset angle alpha.7 This measures the angle between two lines: the first from the midpoint of the femoral neck to the center of the femoral head; and the second from the center of the femoral head to the area where the bone first begins to deviate from the spherical shape of the head. Although first described for use in magnetic resonance imaging, recent evidence has shown that the alpha angle may be a useful measurement on cross-table lateral radiographs.7,8 The accuracy and reproducibility of this measurement on plain film is unknown. While the observation has been made that cartilage and labral injuries occur frequently in hips with cam lesions, little is known about the relationship between the size of the cam lesion and the extent of the pathology. The purpose of this study was to examine the relationship between cam lesions, as measured radiographically by the offset angle alpha, and the presence of cartilage damage, labral damage, and decreased hip range of motion in patients undergoing hip arthroscopy for signs and symptoms of FAI. We hypothesize that patients with a higher offset angle alpha would have greater cartilage damage compared to patients with lower angles.
lesion.5 The cohort therefore included patients with persistent hip pain despite conservative therapy who had radiologic evidence of FAI, as well as intraoperative diagnosis of cam lesions and/or pincer lesions, with the majority demonstrating a combination of the two subtypes. Each patient had a cross-table lateral radiograph performed preoperatively with the leg held in 15° of internal rotation. Patients were excluded from the study if the film was of insufficient quality to clearly see the bony margins to make accurate measurements or if the film showed excessive hip external rotation. Excessive external rotation was defined by the anterior border of the greater trochanter lying behind the anterior femoral shaft. Other exclusion criteria included severe osteoarthritis with complete loss of joint space, dysplasia, avascular necrosis, or Legg-Calvé-Perthes disease. Preoperative alpha angles were measured on the cross-table lateral view by using a digital goniometer (OfficePACS; Stryker Imaging, Flower Mound, TX). Two independent physicians made repeated measurements in order to calculate the intraobserver reliability, and a third physician was added to calculate interobserver reliability. Each observer was blinded to patient information or diagnosis. For each measurement, a spherical template was overlaid on the femoral head to determine the center of the head and to draw a corresponding circle around the circumference of the femoral head. Starting from the first point where any bone deviated from outside this circle, a line was drawn to the center of the femoral head. A second line was drawn from the midpoint of the femoral neck to the center of the femoral head. The angle subtended between these lines was measured as the offset angle alpha (Fig 1).7
METHODS From March 2005 to December 2005, a consecutive series of 102 amateur and professional athletes demonstrating signs and symptoms consistent with FAI underwent arthroscopic hip surgery by the senior author (M.J.P.). Patients typically complained of hip and groin pain that limited activity, had a positive anterior impingement test, and showed classic radiographic evidence for FAI in the form of a cam or pincer
FIGURE 1. The alpha angle is measured by defining the center of the head with a circular template and then drawing the circumference of a circle. One line is drawn from the point at which bone deviates from the circle toward the center of the head; the second is drawn from the center of the isthmus of the femoral neck to the center of the head.
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI TABLE 1.
Intraobserver Interobserver
RESULTS
Reliability of the Measurement of Offset Angle Alpha ICC
95% CI
SEM
0.78 0.5230
0.67-0.85 0.25-0.69
5.3 10.2
Abbreviations: CI, confidence interval; ICC, intraclass correlation coefficient; SEM, standard error of the measurement.
Prospective range of motion data were recorded preoperatively for all patients with a manual goniometer. Hip flexion, adduction, and abduction were measured with the patient in the supine position. The internal and external rotation profiles were measured with the patient in the prone position. Surgical pathology at the time of arthroscopy was prospectively recorded. The presence of chondral lesions on the acetabulum or femoral head and the condition of the labrum were recorded. Full-thickness chondral delamination was noted, along with the size of chondral defects in reference to an arthroscopic probe. Labral tears were classified as intrasubstance or detached at the base. Statistical analysis was performed using SPSS software (version 11; SPSS, Chicago, IL). All reported P values are two-tailed with an alpha level of 0.05 indicating statistical significance. Descriptive statistics (arithmetic mean, standard deviation [SD], and range) were calculated using standard formulas. Alpha angle was normally distributed based on the onesample Komogorov–Smirnov test (P ⬎ .05). Comparison of continuous variables by binary categorical variables was performed using the independent sample t test, and for multiple (⬎2) categorical variables, using one-way analysis of variance. Comparison of two continuous variables was performed using the Pearson correlation coefficient. The reliability (reproducibility) of the measurement technique was examined with the intraclass correlation coefficient (ICC) to express both interobserver reliability (measurements made by different observers) and intraobserver reliability (measurements repeated at different points in time by the same observer) for repeat measurements. To aid in the interpretation, it has been previously categorized as follows: ICC less than 0.20, “slight agreement”; 0.21 to 0.40, “fair agreement”; 0.41 to 0.60, “moderate agreement”; 0.61 to 0.80 “substantial agreement”; and more than 0.80, “almost perfect agreement.”
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Of the 102 patients with available radiographs, 82 patients (47 men, 35 women), with an average age of 25 years (range, 12 to 55), met the study criteria. The intraobserver reliability of the offset angle alpha measurements showed substantial agreement (ICC ⫽ 0.776), while the interobserver reliability had moderate agreement (ICC ⫽ 0.523; Table 1). The average offset angle alpha for the entire cohort was 53.9° (range, 29° to 80°). Men had higher offset angle (57° [range, 38° to 79°]) compared with women (48° [range, 29° to 80°]; P ⫽ .001). Offset angle alpha was not correlated with age. Seventy-nine percent of patients undergoing arthroscopy had chondral defects noted at the time of surgery (Table 2). Patients with chondral defects on the acetabulum had a significantly higher offset angle alpha (55° [range, 29° to 80°]) than patients without chondral defects (49° [range, 38° to 72°]; P ⫽ .044) (Table 3). There was a significant relationship between grade of acetabular cartilage and degrees of alpha angle (P ⫽ .047). Hips with grade IV acetabular chondral defects had 10° greater alpha angle than patients with no acetabular defects. Patients who had large (⬎1.5 cm) acetabular chondral lesions had an offset angle alpha 14° higher (63° [range, 35° to 76°]) than patients without chondral defects (P ⫽ .029). Full-thickness delamination of the acetabular cartilage also occurred with higher offset angles (60° [range, 35° to 80°] v 51° [range, 29° to 75°]; P ⫽ .034). Labral pathology occurred in patients who had a higher offset angle alpha (56° [range, 29° to 80°] v 50° [range, 34° to 79°]; P ⫽ .027). Tears through the base of the labrum occurred in patients with higher offset angles (57° [range, 29° to 80°] v 51° [range, 34° to 79°]; P ⫽ .016). Patients who tore their labrum through the base were more likely to have a medium to large acetabular chondral defect (P ⫽ .001). Midsubstance labral tears, in contrast, occurred at lower offset angle alpha (50°) and were not associated with
TABLE 2.
Outerbridge Classification of Chondral Defects
Grade
No. of Femoral Defects
No. of Acetabular Defects
0 1 2 3 4
36 22 10 6 8
19 13 15 8 27
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TABLE 3.
Hip Damage Versus Offset Angle Alpha
Area of Damage
Mean Offset Angle Alpha (Range)
Acetabulum No cartilage defect Cartilage defect (any size) Large defects (⬎1.5 cm) No cartilage delamination Full-thickness delamination Labrum No labral tear Labral tears Midsubstance tear Labral base tear Femoral head No cartilage defect Cartilage defect (any size)
49° (38°-72°) 55° (29°-80°) 63° (35°-76°) 51° (29°-75°) 60° (35°-80°) 50° (34°-79°) 56° (29°-80°) 51° (34°-79°) 57° (29°-80°) 54° (29°-75°) 57° (35°-80°)
medium to large acetabular chondral defects (not significant). Offset angle alpha was not related to the presence of femoral head defects (not significant). However, medium to large femoral chondral defects (⬎0.5 cm but ⬍1.5 cm) occurred more often when patients had medium to large acetabular defects (P ⫽ .001). Decreased range of motion commonly occurred in patients with higher offset angle alpha (Table 4). DISCUSSION The main findings of this study were that larger cam lesions, as measured by the offset angle alpha, were associated with larger areas of acetabular cartilage damage, acetabular delamination, and labral base detachment. These findings are consistent with the pathogenesis proposed by Ganz et al.6 Furthermore, our findings confirm our hypothesis that patients with a higher offset angle alpha had greater cartilage damage, labral injury, and decreased range of motion. The cartilage and labral damage patterns observed in this study likely represent a continuum of injury that starts at the acetabular edge and progresses. Initially, a mild chondrolabral injury at the edge of the acetabulum (Fig 2A) is induced by shear stresses from the asymmetric cam lesion entering into the joint. The injury progresses as the cartilage at the acetabular rim fails (Fig 2B), either at the chondrolabral junction or by undersurface delamination. Loss of continuity at the chondrolabral junction (Fig 2C) exposes the labrum to further tearing through the base while the cartilage loses its inherent stability against further shear forces. As this acetabular cartilage injury enlarges (Fig 2D), reciprocal damage to the femoral
head may be induced by contact with the irregular acetabular surface leading to the end result of an osteoarthritic hip. It is unclear at what point this continuum of injury progresses beyond a recoverable injury pattern. The mere presence of a cam lesion alone does not appear to be sufficient to predict injury, as we have clinically observed many patients with bilateral cam lesions with only one symptomatic hip. The prevalence of asymptomatic cam lesions in a normal population is currently unknown. We hypothesize that the presence of a large cam lesion creates an “at-risk” hip with a negative mechanical environment that predisposes a mildly injured hip to rapidly decompensate further down the previously described continuum of injury. An “at-risk” hip may be pushed beyond a recoverable injury pattern by any number of factors including the size of the cam lesion, previous hip injury, length of symptoms, hip activities performed, frequency of participation, acetabular retroversion, and the resiliency of an individual’s cartilage to injury. Further research is needed to determine the prevalence and natural history of asymptomatic cam lesions as well as the relative contribution of these variables to the development of intra-articular hip damage. In a recent study by Stahelin et al.,9 they looked at the accuracy of arthroscopic restoration of the femoral head–neck offset. Their group was able to accurately restore anatomic offset by improving the alpha angle by a mean of 21°, from 75° to 54°.9 The patients in this cohort had significant gains in range of motion and showed significant improvement in outcome measures at early follow-up after hip arthroscopy for femoral head–neck abnormality restoration.9 In our study, higher alpha angles occurred in males. This finding is consistent with clinical reports by other authors who characterize the cam lesion as a primary problem of young, active males.10,11 Although the
TABLE 4.
Correlation of Deficit in Range of Motion and Increasing Offset Angle Alpha
Motion
Correlation Coefficient (R)
Significance (P)
Internal rotation External rotation Abduction Adduction Flexion
⫺0.263
.023*
⫺0.314 ⫺0.259 ⫺0.214 ⫺0.213
.007* .022* .116 .060
*Denotes statistical significance.
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI
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FIGURE 2. Intraoperative view at arthroscopy demonstrating classic damage to the labrochondral junction secondary to cam impingement. (A) The labrum is detached from its junction with the cartilage, and the cartilage is beginning to peel up from the edge like a carpet. (B) Labral detachment at the base, with progressive delamination injury to the cartilage. The shaver is pushing on the cartilage edge and shows a positive “wave sign.” (C) The delaminated cartilage begins to fragment and separate, leaving an area of grade 4 loss. (D) More advanced damage secondary to cam impingement. The acetabular cartilage is absent and the labral base is completely detached. Note the reciprocal changes developing on the femoral head cartilage. Abbreviations: C, cartilage; L, labrum.
reason for the difference in sex distribution is unknown, we hypothesize that it may be related to subclinical physeal injury related to early aggressive sports activity in males. Patients participating in certain sports, such as hockey, football, soccer, rugby, martial arts, and tennis, are more likely to present with cam lesions and may be considered at risk for the development of cam sequelae.11,12 Further studies into
this hypothesis need to be undertaken, because the results will have significant implications for hip screening in youth athletic programs. Of note, it is important to recognize that a new method for quantifying femoral head–neck asphericity has been evaluated. Gosvig et al.13 discussed the triangular index for assessment of the asphericity of the bony abnormality at the femoral head–neck. They
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concluded that the triangular index is a useful method in quantifying femoral head–neck abnormality on anteroposterior pelvic radiographs.13 In addition to early osteoarthritis, several authors have described a loss of motion, particularly internal rotation, in patients with cam impingement.2,7,14,15 It has not been clear whether more extensive lesions result in more severe motion deficits. In this study, subtle side-to-side decreases in internal and external rotation and abduction were detected in patients with higher offset angles. Therefore, asymmetric hip motion found on physical examination in a symptomatic individual should raise suspicion for cam-type FAI, particularly in the presence of a positive anterior impingement test. We believe that measurements of flexion were not significantly altered by the offset angle alpha because the leg was allowed to freely rotate in flexion. Our clinical experience suggests that most patients with cam lesions do have an alteration in hip flexion; however, this would only be detected by statically limiting the obligate external rotation. Holm et al.16 examined the reliability of goniometric measurements for hip range of motion in patients with osteoarthritis.16 They looked at three teams of physiotherapists using goniometric measurements. They found excellent reproducibility for range of motion measurements; however, they did find a difference between measurements recorded by one person when compared with measurements measured by two people.16 Our range of motion measurements were made by two physical therapists using a goniometer. A value for the “normal” offset angle alpha has not been well defined. Our reliability data suggest that it is useful for measuring differences from one patient to the next; some caution, however, should be observed in generalizing the absolute values because of wide interobserver differences. This is likely related to each physician using a different measurement point on the wide transition from spherical to aspherical hip. Nötzli et al.7 examined an asymptomatic population of hip patients and found an average offset angle alpha of 42° on magnetic resonance imaging scan, while Meyer et al.8 examined offset angles on plain radiographs and found an average normal angle of 50°. In this study, patients who did not have cartilage lesions had an average offset angle alpha of 49°. Synthesis of these 3 end-points suggests that a “normal” offset angle alpha could be defined as less than 50°, but this conclusion is limited by the previously noted poor interobserver reliability. One limitation inherent to the offset angle alpha measurement is that it only detects the location of the abnormal bone. It does not measure the height of the
bump, which may also be a contributing factor. In patients with large and convex lesions, measurements similar to those described by Ito et al.17 must be performed to evaluate the extent of convexity. Our decision to exclude patients with complete loss of joint space could have potentially led to an underestimation of our findings, particularly in the larger lesions. However, these radiographs often had large secondary osteophytes that made accurate measurement of alpha angles difficult, and to maintain accuracy of the alpha angle measurements, we opted to exclude these patients. The cross-table lateral view is a cost efficient, albeit slightly inaccurate, way to measure offset angle alpha that is available in the clinic setting. There are, however, a small number of patients who have primarily lateral-based cam lesions that would only be seen on an anteroposterior radiograph, raising the possibility of a small underestimation of cam lesion size in some patients in this study. In clinical practice, the anteroposterior and cross-table lateral views should always be obtained together. A recent study evaluated several plain radiograph views, including the cross-table lateral, for accuracy in measuring the alpha angle, and found most views to be sensitive.8 Clohisy et al.18 recently looked at the accuracy of the frog-leg lateral radiograph to assess femoral head–neck offset abnormality.18 Their group concluded that the frog-leg lateral radiograph provided accurate visualization of the femoral head–neck bony abnormality in patients with cam type FAI.18 Importantly, the cross-table lateral view was significantly affected by hip rotation with a higher chance of missing the cam lesion and getting a false negative when the hip was in external rotation. For this reason, we chose to exclude any radiograph that appeared externally rotated to improve measurement accuracy. Because our patient population is biased toward athletes, there is the possibility that our pathologic observations may have been skewed towards more severe damage. However, these pathologic findings2,6 and offset angle alpha measurements7,8 are consistent with those from other reported studies, and we feel they should still be valid when applied to a general population of patients. CONCLUSIONS Cam-type FAI, as measured by an increased offset angle alpha, is associated with increasing chondral damage, labral injury, and decreased range of motion. Offset angle alpha is a useful measurement when
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI performed on the cross-table lateral radiograph that reliably detects differences in cam lesion size from one patient to the next. REFERENCES 1. Solomon L. Patterns of osteoarthritis of the hip. J Bone Joint Surg Br 1976;58:176-183. 2. Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoroacetabular impingement: A cause for osteoarthritis of the hip. Clin Orthop Relat Res 2003;417:112-120. 3. Wenger DE, Kendell KR, Miner MR, Trousdale RT. Acetabular labral tears rarely occur in the absence of bony abnormalities. Clin Orthop Relat Res 2004;426:145-150. 4. Philippon MJ, Stubbs AJ, Schenker ML, Maxwell RB, Ganz R, Leunig M. Arthroscopic management of femoroacetabular impingement: Osteoplasty technique and literature review. Am J Sports Med 2007;35:1571-1580. 5. Lavigne M, Parvizi J, Beck M, Siebenrock KA, Ganz R, Leunig M. Anterior femoroacetabular impingement: Part I. Techniques of joint preserving surgery. Clin Orthop Relat Res 2004;418:61-66. 6. Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: Femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br 2005;87:1012-1018. 7. Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br 2002;84:556-560. 8. Meyer DC, Beck M, Ellis T, Ganz R, Leunig M. Comparison of six radiographic projections to assess femoral head/neck asphericity. Clin Orthop Relat Res 2006;445:181-185. 9. Stahelin L, Stahelin T, Jolles BM, Herzog RF. Arthroscopic offset restoration in femoroacetabular impingement cam im-
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pingement: Accuracy and early clinical outcome. Arthroscopy 2008;24:51-57. Beck M, Leunig M, Parvizi J, Boutier V, Wyss D, Ganz R. Anterior femoroacetabular impingement: Part II. Midterm results of surgical treatment. Clin Orthop Relat Res 2004;418: 67-73. Desmarais Y, Lequesne M. La han Zche du sportif. Gaz Med Fr 1979;86:2969-2972. Philippon M, Schenker M, Briggs K, Kuppersmith D. Femoroacetabular impingement in 45 professional athletes: Associated pathologies and return to sport following arthroscopic decompression. Knee Surg Sports Traumatol Arthrosc 2007; 15:908-914. Gosvig KK, Jacobsen S, Palm H, Sonne-Holm S, Magnusson E. A new radiological index for assessing asphericity of the femoral head in cam impingement. J Bone Joint Surg Br 2007;89:1309-1316. Beall DP, Sweet CF, Martin HD, et al. Imaging findings of femoroacetabular impingement syndrome. Skeletal Radiol 2005;34:691-701. Jager M, Wild A, Westhoff B, Krauspe R. Femoroacetabular impingement caused by a femoral osseous head-neck bump deformity: Clinical, radiological, and experimental results. J Orthop Sci 2004;9:256-263. Holm I, Bolstad B, Lutken T, Ervik A, Rokkum M, Steen H. Reliability of goniometric measurements and visual estimates of hip ROM in patients with osteoarthrosis. Physiother Res Int 2000;5:241-248. Ito K, Minka MA, Leunig M, Werlen S, Ganz R. Femoroacetabular impingement and the cam-effect. A MRI-based quantitative anatomical study of the femoral head-neck offset. J Bone Joint Surg Br 2001;83:171-176. Clohisy JC, Nunley RM, Otto RJ, Schoenecker PL. The frog-leg lateral radiograph accurately visualized hip cam impingement abnormalities. Clin Orthop Relat Res 2007; 462:115-121.
Purpose: The purpose of this study was to examine the relationship between the size of cam lesions and the presence of cartilage damage, labral damage, or changes in range of motion in the hips with signs and symptoms of femoroacetabular impingement (FAI). Methods: Cross-table lateral radiographs were available for 102 consecutive patients presenting with signs and symptoms of FAI. Radiographs with excessive external rotation, dysplasia, severe arthritis, avascular necrosis, or Legg-Calvé-Perthes syndrome were excluded, leaving 82 patients available for analysis (47 men, 35 women; average age, 25 yr [range, 12 to 55 yr]). Offset angle alpha was measured from the films with a digital goniometer. Patients subsequently underwent hip arthroscopy and the surgical findings and hip range of motion were prospectively recorded. Results: Higher offset angle alpha was associated with the presence of acetabular rim chondral defects (P ⫽ .044) and full-thickness delamination of the acetabular cartilage (P ⫽ .034). Patients with detachment of the base of the labrum had a higher offset angle alpha (P ⫽ .016). Higher offset angle alpha was related to male sex (P ⫽ .001) and decreased range of motion (P ⬍ .05), but not to age. Conclusions: Cam-type FAI, as measured by an increased offset angle alpha, was correlated with increased chondral damage, labral injury, and decreased range of motion. Level of Evidence: Level II, development of diagnostic criteria on basis of consecutive patients with universally applied gold standard. Key Words: Alpha angle—Cam— Femoroacetabular impingement—Hip arthroscopy—Offset angle alpha.
J
ust under 30% of published cases of hip osteoarthritis are attributed to primary osteoarthritis in which the cause is unknown.1 Significant recent advances in our understanding of hip pathology have led to the hypothesis that small alterations in hip morphology can cause motion-induced mechanical damage over time.2-4 The term femoroacetabular impingement (FAI) has been coined to describe these morphologic
From Cedar Valley Medical Associates (T.L.J.), Waterloo, Iowa, and Steadman Hawkins Research Foundation (M.L.S., K.K.B., M.J.P.), Vail, Colorado, U.S.A. Supported in part by a grant from Smith & Nephew. Address correspondence and reprint requests to Marc J. Philippon, M.D., Steadman Hawkins Research Foundation, Attn: Clinical Research, 181 W. Meadow Dr., Ste. 1000, Vail, CO 81657, U.S.A. E-mail: [email protected] © 2008 by the Arthroscopy Association of North America 0749-8063/08/2406-7302$34.00/0 doi:10.1016/j.arthro.2008.01.010
alterations, which have been implicated as a potential cause of primary hip osteoarthritis.2 There are two described subtypes of hip impingement: cam and pincer.5 Cam impingement results from excessive bone at the femoral head–neck junction that extends beyond the normal sphericity of the femoral head, or when there is a flattening and loss of the normal offset between the femoral head and neck.5 Pincer impingement results from excessive bone along the acetabular rim or relative acetabular retroversion.4 More than 80% of patients presenting with FAI are reported to have a combination of the two subtypes.6 The etiologies of these morphologic lesions are not well understood. Observations made during open surgical treatment for FAI have led to the hypothesis that FAI may be a cause of early osteoarthritis.2,6 Surgical findings suggest that cam impingement, in particular, can lead to significant chondral and labral injuries when the areas of excess bone on the proximal femoral neck are
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forced into the joint with motion. This incongruence is thought to lead to increased friction and a concentration of shear forces parallel to the acetabular rim cartilage and labral structures. Ganz et al.2 have suggested that as the area of femoral incongruence enters into the acetabulum, the labrum is pushed up and the shear stresses are concentrated on the labral– chondral junction, leading to a separation. The extent of the chondral damage has been observed to vary from small areas of chondral delamination to larger, fullthickness lesions as the base of the cartilage is sheared and pulled up like the edge of a carpet. Labral detachment is also a frequent finding and may occur at the base of the labrum near the labral– chondral junction. Radiographic methods for defining cam impingement have advanced in the last few years. One instrument for quantifying the extent of a cam lesion is the offset angle alpha.7 This measures the angle between two lines: the first from the midpoint of the femoral neck to the center of the femoral head; and the second from the center of the femoral head to the area where the bone first begins to deviate from the spherical shape of the head. Although first described for use in magnetic resonance imaging, recent evidence has shown that the alpha angle may be a useful measurement on cross-table lateral radiographs.7,8 The accuracy and reproducibility of this measurement on plain film is unknown. While the observation has been made that cartilage and labral injuries occur frequently in hips with cam lesions, little is known about the relationship between the size of the cam lesion and the extent of the pathology. The purpose of this study was to examine the relationship between cam lesions, as measured radiographically by the offset angle alpha, and the presence of cartilage damage, labral damage, and decreased hip range of motion in patients undergoing hip arthroscopy for signs and symptoms of FAI. We hypothesize that patients with a higher offset angle alpha would have greater cartilage damage compared to patients with lower angles.
lesion.5 The cohort therefore included patients with persistent hip pain despite conservative therapy who had radiologic evidence of FAI, as well as intraoperative diagnosis of cam lesions and/or pincer lesions, with the majority demonstrating a combination of the two subtypes. Each patient had a cross-table lateral radiograph performed preoperatively with the leg held in 15° of internal rotation. Patients were excluded from the study if the film was of insufficient quality to clearly see the bony margins to make accurate measurements or if the film showed excessive hip external rotation. Excessive external rotation was defined by the anterior border of the greater trochanter lying behind the anterior femoral shaft. Other exclusion criteria included severe osteoarthritis with complete loss of joint space, dysplasia, avascular necrosis, or Legg-Calvé-Perthes disease. Preoperative alpha angles were measured on the cross-table lateral view by using a digital goniometer (OfficePACS; Stryker Imaging, Flower Mound, TX). Two independent physicians made repeated measurements in order to calculate the intraobserver reliability, and a third physician was added to calculate interobserver reliability. Each observer was blinded to patient information or diagnosis. For each measurement, a spherical template was overlaid on the femoral head to determine the center of the head and to draw a corresponding circle around the circumference of the femoral head. Starting from the first point where any bone deviated from outside this circle, a line was drawn to the center of the femoral head. A second line was drawn from the midpoint of the femoral neck to the center of the femoral head. The angle subtended between these lines was measured as the offset angle alpha (Fig 1).7
METHODS From March 2005 to December 2005, a consecutive series of 102 amateur and professional athletes demonstrating signs and symptoms consistent with FAI underwent arthroscopic hip surgery by the senior author (M.J.P.). Patients typically complained of hip and groin pain that limited activity, had a positive anterior impingement test, and showed classic radiographic evidence for FAI in the form of a cam or pincer
FIGURE 1. The alpha angle is measured by defining the center of the head with a circular template and then drawing the circumference of a circle. One line is drawn from the point at which bone deviates from the circle toward the center of the head; the second is drawn from the center of the isthmus of the femoral neck to the center of the head.
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI TABLE 1.
Intraobserver Interobserver
RESULTS
Reliability of the Measurement of Offset Angle Alpha ICC
95% CI
SEM
0.78 0.5230
0.67-0.85 0.25-0.69
5.3 10.2
Abbreviations: CI, confidence interval; ICC, intraclass correlation coefficient; SEM, standard error of the measurement.
Prospective range of motion data were recorded preoperatively for all patients with a manual goniometer. Hip flexion, adduction, and abduction were measured with the patient in the supine position. The internal and external rotation profiles were measured with the patient in the prone position. Surgical pathology at the time of arthroscopy was prospectively recorded. The presence of chondral lesions on the acetabulum or femoral head and the condition of the labrum were recorded. Full-thickness chondral delamination was noted, along with the size of chondral defects in reference to an arthroscopic probe. Labral tears were classified as intrasubstance or detached at the base. Statistical analysis was performed using SPSS software (version 11; SPSS, Chicago, IL). All reported P values are two-tailed with an alpha level of 0.05 indicating statistical significance. Descriptive statistics (arithmetic mean, standard deviation [SD], and range) were calculated using standard formulas. Alpha angle was normally distributed based on the onesample Komogorov–Smirnov test (P ⬎ .05). Comparison of continuous variables by binary categorical variables was performed using the independent sample t test, and for multiple (⬎2) categorical variables, using one-way analysis of variance. Comparison of two continuous variables was performed using the Pearson correlation coefficient. The reliability (reproducibility) of the measurement technique was examined with the intraclass correlation coefficient (ICC) to express both interobserver reliability (measurements made by different observers) and intraobserver reliability (measurements repeated at different points in time by the same observer) for repeat measurements. To aid in the interpretation, it has been previously categorized as follows: ICC less than 0.20, “slight agreement”; 0.21 to 0.40, “fair agreement”; 0.41 to 0.60, “moderate agreement”; 0.61 to 0.80 “substantial agreement”; and more than 0.80, “almost perfect agreement.”
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Of the 102 patients with available radiographs, 82 patients (47 men, 35 women), with an average age of 25 years (range, 12 to 55), met the study criteria. The intraobserver reliability of the offset angle alpha measurements showed substantial agreement (ICC ⫽ 0.776), while the interobserver reliability had moderate agreement (ICC ⫽ 0.523; Table 1). The average offset angle alpha for the entire cohort was 53.9° (range, 29° to 80°). Men had higher offset angle (57° [range, 38° to 79°]) compared with women (48° [range, 29° to 80°]; P ⫽ .001). Offset angle alpha was not correlated with age. Seventy-nine percent of patients undergoing arthroscopy had chondral defects noted at the time of surgery (Table 2). Patients with chondral defects on the acetabulum had a significantly higher offset angle alpha (55° [range, 29° to 80°]) than patients without chondral defects (49° [range, 38° to 72°]; P ⫽ .044) (Table 3). There was a significant relationship between grade of acetabular cartilage and degrees of alpha angle (P ⫽ .047). Hips with grade IV acetabular chondral defects had 10° greater alpha angle than patients with no acetabular defects. Patients who had large (⬎1.5 cm) acetabular chondral lesions had an offset angle alpha 14° higher (63° [range, 35° to 76°]) than patients without chondral defects (P ⫽ .029). Full-thickness delamination of the acetabular cartilage also occurred with higher offset angles (60° [range, 35° to 80°] v 51° [range, 29° to 75°]; P ⫽ .034). Labral pathology occurred in patients who had a higher offset angle alpha (56° [range, 29° to 80°] v 50° [range, 34° to 79°]; P ⫽ .027). Tears through the base of the labrum occurred in patients with higher offset angles (57° [range, 29° to 80°] v 51° [range, 34° to 79°]; P ⫽ .016). Patients who tore their labrum through the base were more likely to have a medium to large acetabular chondral defect (P ⫽ .001). Midsubstance labral tears, in contrast, occurred at lower offset angle alpha (50°) and were not associated with
TABLE 2.
Outerbridge Classification of Chondral Defects
Grade
No. of Femoral Defects
No. of Acetabular Defects
0 1 2 3 4
36 22 10 6 8
19 13 15 8 27
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TABLE 3.
Hip Damage Versus Offset Angle Alpha
Area of Damage
Mean Offset Angle Alpha (Range)
Acetabulum No cartilage defect Cartilage defect (any size) Large defects (⬎1.5 cm) No cartilage delamination Full-thickness delamination Labrum No labral tear Labral tears Midsubstance tear Labral base tear Femoral head No cartilage defect Cartilage defect (any size)
49° (38°-72°) 55° (29°-80°) 63° (35°-76°) 51° (29°-75°) 60° (35°-80°) 50° (34°-79°) 56° (29°-80°) 51° (34°-79°) 57° (29°-80°) 54° (29°-75°) 57° (35°-80°)
medium to large acetabular chondral defects (not significant). Offset angle alpha was not related to the presence of femoral head defects (not significant). However, medium to large femoral chondral defects (⬎0.5 cm but ⬍1.5 cm) occurred more often when patients had medium to large acetabular defects (P ⫽ .001). Decreased range of motion commonly occurred in patients with higher offset angle alpha (Table 4). DISCUSSION The main findings of this study were that larger cam lesions, as measured by the offset angle alpha, were associated with larger areas of acetabular cartilage damage, acetabular delamination, and labral base detachment. These findings are consistent with the pathogenesis proposed by Ganz et al.6 Furthermore, our findings confirm our hypothesis that patients with a higher offset angle alpha had greater cartilage damage, labral injury, and decreased range of motion. The cartilage and labral damage patterns observed in this study likely represent a continuum of injury that starts at the acetabular edge and progresses. Initially, a mild chondrolabral injury at the edge of the acetabulum (Fig 2A) is induced by shear stresses from the asymmetric cam lesion entering into the joint. The injury progresses as the cartilage at the acetabular rim fails (Fig 2B), either at the chondrolabral junction or by undersurface delamination. Loss of continuity at the chondrolabral junction (Fig 2C) exposes the labrum to further tearing through the base while the cartilage loses its inherent stability against further shear forces. As this acetabular cartilage injury enlarges (Fig 2D), reciprocal damage to the femoral
head may be induced by contact with the irregular acetabular surface leading to the end result of an osteoarthritic hip. It is unclear at what point this continuum of injury progresses beyond a recoverable injury pattern. The mere presence of a cam lesion alone does not appear to be sufficient to predict injury, as we have clinically observed many patients with bilateral cam lesions with only one symptomatic hip. The prevalence of asymptomatic cam lesions in a normal population is currently unknown. We hypothesize that the presence of a large cam lesion creates an “at-risk” hip with a negative mechanical environment that predisposes a mildly injured hip to rapidly decompensate further down the previously described continuum of injury. An “at-risk” hip may be pushed beyond a recoverable injury pattern by any number of factors including the size of the cam lesion, previous hip injury, length of symptoms, hip activities performed, frequency of participation, acetabular retroversion, and the resiliency of an individual’s cartilage to injury. Further research is needed to determine the prevalence and natural history of asymptomatic cam lesions as well as the relative contribution of these variables to the development of intra-articular hip damage. In a recent study by Stahelin et al.,9 they looked at the accuracy of arthroscopic restoration of the femoral head–neck offset. Their group was able to accurately restore anatomic offset by improving the alpha angle by a mean of 21°, from 75° to 54°.9 The patients in this cohort had significant gains in range of motion and showed significant improvement in outcome measures at early follow-up after hip arthroscopy for femoral head–neck abnormality restoration.9 In our study, higher alpha angles occurred in males. This finding is consistent with clinical reports by other authors who characterize the cam lesion as a primary problem of young, active males.10,11 Although the
TABLE 4.
Correlation of Deficit in Range of Motion and Increasing Offset Angle Alpha
Motion
Correlation Coefficient (R)
Significance (P)
Internal rotation External rotation Abduction Adduction Flexion
⫺0.263
.023*
⫺0.314 ⫺0.259 ⫺0.214 ⫺0.213
.007* .022* .116 .060
*Denotes statistical significance.
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI
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FIGURE 2. Intraoperative view at arthroscopy demonstrating classic damage to the labrochondral junction secondary to cam impingement. (A) The labrum is detached from its junction with the cartilage, and the cartilage is beginning to peel up from the edge like a carpet. (B) Labral detachment at the base, with progressive delamination injury to the cartilage. The shaver is pushing on the cartilage edge and shows a positive “wave sign.” (C) The delaminated cartilage begins to fragment and separate, leaving an area of grade 4 loss. (D) More advanced damage secondary to cam impingement. The acetabular cartilage is absent and the labral base is completely detached. Note the reciprocal changes developing on the femoral head cartilage. Abbreviations: C, cartilage; L, labrum.
reason for the difference in sex distribution is unknown, we hypothesize that it may be related to subclinical physeal injury related to early aggressive sports activity in males. Patients participating in certain sports, such as hockey, football, soccer, rugby, martial arts, and tennis, are more likely to present with cam lesions and may be considered at risk for the development of cam sequelae.11,12 Further studies into
this hypothesis need to be undertaken, because the results will have significant implications for hip screening in youth athletic programs. Of note, it is important to recognize that a new method for quantifying femoral head–neck asphericity has been evaluated. Gosvig et al.13 discussed the triangular index for assessment of the asphericity of the bony abnormality at the femoral head–neck. They
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concluded that the triangular index is a useful method in quantifying femoral head–neck abnormality on anteroposterior pelvic radiographs.13 In addition to early osteoarthritis, several authors have described a loss of motion, particularly internal rotation, in patients with cam impingement.2,7,14,15 It has not been clear whether more extensive lesions result in more severe motion deficits. In this study, subtle side-to-side decreases in internal and external rotation and abduction were detected in patients with higher offset angles. Therefore, asymmetric hip motion found on physical examination in a symptomatic individual should raise suspicion for cam-type FAI, particularly in the presence of a positive anterior impingement test. We believe that measurements of flexion were not significantly altered by the offset angle alpha because the leg was allowed to freely rotate in flexion. Our clinical experience suggests that most patients with cam lesions do have an alteration in hip flexion; however, this would only be detected by statically limiting the obligate external rotation. Holm et al.16 examined the reliability of goniometric measurements for hip range of motion in patients with osteoarthritis.16 They looked at three teams of physiotherapists using goniometric measurements. They found excellent reproducibility for range of motion measurements; however, they did find a difference between measurements recorded by one person when compared with measurements measured by two people.16 Our range of motion measurements were made by two physical therapists using a goniometer. A value for the “normal” offset angle alpha has not been well defined. Our reliability data suggest that it is useful for measuring differences from one patient to the next; some caution, however, should be observed in generalizing the absolute values because of wide interobserver differences. This is likely related to each physician using a different measurement point on the wide transition from spherical to aspherical hip. Nötzli et al.7 examined an asymptomatic population of hip patients and found an average offset angle alpha of 42° on magnetic resonance imaging scan, while Meyer et al.8 examined offset angles on plain radiographs and found an average normal angle of 50°. In this study, patients who did not have cartilage lesions had an average offset angle alpha of 49°. Synthesis of these 3 end-points suggests that a “normal” offset angle alpha could be defined as less than 50°, but this conclusion is limited by the previously noted poor interobserver reliability. One limitation inherent to the offset angle alpha measurement is that it only detects the location of the abnormal bone. It does not measure the height of the
bump, which may also be a contributing factor. In patients with large and convex lesions, measurements similar to those described by Ito et al.17 must be performed to evaluate the extent of convexity. Our decision to exclude patients with complete loss of joint space could have potentially led to an underestimation of our findings, particularly in the larger lesions. However, these radiographs often had large secondary osteophytes that made accurate measurement of alpha angles difficult, and to maintain accuracy of the alpha angle measurements, we opted to exclude these patients. The cross-table lateral view is a cost efficient, albeit slightly inaccurate, way to measure offset angle alpha that is available in the clinic setting. There are, however, a small number of patients who have primarily lateral-based cam lesions that would only be seen on an anteroposterior radiograph, raising the possibility of a small underestimation of cam lesion size in some patients in this study. In clinical practice, the anteroposterior and cross-table lateral views should always be obtained together. A recent study evaluated several plain radiograph views, including the cross-table lateral, for accuracy in measuring the alpha angle, and found most views to be sensitive.8 Clohisy et al.18 recently looked at the accuracy of the frog-leg lateral radiograph to assess femoral head–neck offset abnormality.18 Their group concluded that the frog-leg lateral radiograph provided accurate visualization of the femoral head–neck bony abnormality in patients with cam type FAI.18 Importantly, the cross-table lateral view was significantly affected by hip rotation with a higher chance of missing the cam lesion and getting a false negative when the hip was in external rotation. For this reason, we chose to exclude any radiograph that appeared externally rotated to improve measurement accuracy. Because our patient population is biased toward athletes, there is the possibility that our pathologic observations may have been skewed towards more severe damage. However, these pathologic findings2,6 and offset angle alpha measurements7,8 are consistent with those from other reported studies, and we feel they should still be valid when applied to a general population of patients. CONCLUSIONS Cam-type FAI, as measured by an increased offset angle alpha, is associated with increasing chondral damage, labral injury, and decreased range of motion. Offset angle alpha is a useful measurement when
OFFSET ANGLE ALPHA AND HIP CHONDRAL INJURY IN FAI performed on the cross-table lateral radiograph that reliably detects differences in cam lesion size from one patient to the next. REFERENCES 1. Solomon L. Patterns of osteoarthritis of the hip. J Bone Joint Surg Br 1976;58:176-183. 2. Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoroacetabular impingement: A cause for osteoarthritis of the hip. Clin Orthop Relat Res 2003;417:112-120. 3. Wenger DE, Kendell KR, Miner MR, Trousdale RT. Acetabular labral tears rarely occur in the absence of bony abnormalities. Clin Orthop Relat Res 2004;426:145-150. 4. Philippon MJ, Stubbs AJ, Schenker ML, Maxwell RB, Ganz R, Leunig M. Arthroscopic management of femoroacetabular impingement: Osteoplasty technique and literature review. Am J Sports Med 2007;35:1571-1580. 5. Lavigne M, Parvizi J, Beck M, Siebenrock KA, Ganz R, Leunig M. Anterior femoroacetabular impingement: Part I. Techniques of joint preserving surgery. Clin Orthop Relat Res 2004;418:61-66. 6. Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: Femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br 2005;87:1012-1018. 7. Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br 2002;84:556-560. 8. Meyer DC, Beck M, Ellis T, Ganz R, Leunig M. Comparison of six radiographic projections to assess femoral head/neck asphericity. Clin Orthop Relat Res 2006;445:181-185. 9. Stahelin L, Stahelin T, Jolles BM, Herzog RF. Arthroscopic offset restoration in femoroacetabular impingement cam im-
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