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. 2012;32:1–8.

CAM-Type Impingement in the Ankle

Ned Amendola 1, Newhoff Drew 1, Tanawat Vaseenon 2, John Femino 1, Yuki Tochigi 1, Phinit Phisitkul 1
PMCID: PMC3565388  PMID: 23576914

Abstract

Background

Anterior ankle impingement with and without ankle osteoarthritis (OA) is a common condition. Bony impingement between the distal tibia and talus aggravated by dorsiflexion has been well described. The etiology of these impingement lesions remains controversial. This study describes a cam-type impingement of the ankle, in which the sagittal contour of the talar dome is a non-circular arc, causing pathologic contact with the anterior aspect of the tibial plafond during dorsiflexion, leading to abnormal ankle joint mechanics by limiting dorsiflexion.

Methods

A group of 269 consecutive adult patients from the University of Iowa Hospitals and Clinics who were treated for anterior bony impingement syndrome were evaluated as the study population. As a control group, 41 patients without any evidence of impingement or arthrosis were evaluated. Standardized standing lateral ankle radiographs were evaluated to determine the contour of the head/neck relationship in the talus. Two investigators made all the radiographic measurements and intra- and inter-observer reliability were measured.

Results

34% of patients were found to have some anterior extension of the talar dome creating a loss of the normal concavity at the dorsal medial talar neck. A group of 36 patients (13%) were identified as having the most severe cam deformity in order to assess any correlation with coexisting radiographic abnormalities. In these patients, a cavo-varus foot type was more commonly observed. Comparison with a control group showed much lower rates of anterior-medial cam-type deformity of the talus.

Conclusions

Cam type impingement of the ankle is likely a distinct form of bony impingement of the ankle secondary to a morphological talar bony abnormality. Based on the findings of this study, this form of impingement may be related to a cavovarus foot type. In addition, there may be long term implications in the development of ankle OA.

Level of Evidence

Level III

Introduction

Anterior ankle impingement syndrome presents with pain during ankle dorsiflexion. This is thought to be due to hypertrophy of soft tissues or bone surrounding the tibiotalar joint.1 This condition is commonly thought to develop in response to repetitive trauma and torsional stresses to the joint, having been originally identified by Morris and McMurray in European footballers.2, 3, 4 Repetitive forced dorsiflexion can result in anterior compression of the tibiotalar joint, and repeated direct contact at the tibiotalar junction has been shown to induce osteophyte development.2, 5 Over time, attempted repair, including fibrosis and fibrocartilage proliferation, leads to osteophyte development2. These osteophytes can limit ankle dorsiflexion and increase irritation of nearby soft tissues.2 Radiographs are commonly obtained for these patients and are usually diagnostic.1 Conservative approaches to treatment, including rest, physical therapy, and anti-inflammatory treatment, are effective for many patients.1, 6 Surgical treatment for these patients is reserved for cases where conservative measures fail.1, 6 Both open and arthroscopic debridement procedures have high success rates for treating anterior ankle impingment.6, 7, 8, 9 Most patients return to full activity, including athletics, following either type of treatment.1, 7 Tol et al. found recurrence of some osteophytes in two-thirds of patients with grade-I OA following arthroscopic debridement for impingement.7 A possible cause of failure in these patients is a more significant or different type of impingement.

This study attempts to identify a cam-type impingement of the ankle, similar to what has been described in the hip femoral neck,10 in which the sagittal contour of the talar dome forms a non-circular arc with an anterior flattening that causes loss of the normal concavity of the talar neck and pathologic contact with the anterior aspect of the tibial plafond in dorsiflexion, and abnormal loading of the talar dome cartilage (Fig. 1). This anatomic relationship could also lead to the formation of reactive osteophytes and soft tissue hypertrophy but specifically differs from anterior impingement syndrome in that there appears to be an underlying anatomical bony deformity of the talar body-neck junction (Fig. 2). Sarrafian described the normal talus as having a medial extension facet of the talar body articular surface in 36% of 100 specimens.11

Figure 1. Lateral xray of a patient with cam impingement without any typical osteophytes or OA of the ankle ( non spherical nature of the talar dome causing impingement).

Figure 1

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Figure 2. Patient with advanced OA of the ankle with what appears to be chronic cam impingement of the ankle (the non spherical nature of the talar dome with impingement and extrusion of the talus from the mortise).

Figure 2

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A parallel scenario in cam impingement of the hip has been previously identified by Notzli et al.10 In this condition, an aspherical portion of the femoral neck causes pathologic contact with the acetabulum, leading to decreased mobility and pain on internal rotation.12, 13 Acetabular morphology in this condition is normal.13 Contact at the femoral neck with cam impingement causes inflammation of the surrounding tissues and mimics typical impingment.10, 12 Cam impingement is found in 40% of patients who develop osteoarthritis at the hip and typically leads to progressive degeneration of joint stability.12 Cam impingement of the hip is generally treated with arthroscopic or open surgery to remove the skeletal deformities causing impingement.14 Patients who undergo surgery to resect the cam lesion have better range-of-motion and less pain than those who undergo physical therapy alone.14 Although long term results are not available yet, it is believed that this procedure is also beneficial in preventing future development of arthritis in the joint.12

Although ankle impingement has been a welldescribed condition, there is a notable incidence of failure and recurrence with current surgical treatment of anterior impingement. This may be in part due to the surgical removal of osteophytes only without addressing the bony deformity. Medial impingement has been described in gymnasts with some unique characteristics.15 Hamilton may have described this as the “unrecognized osteophyte.”15 This form of cam impingement and ankle deformity has been observed by the senior author (AA) in numerous cases. We feel that the impingement we describe in this paper is a unique form of bony ankle impingement which has not previously been identified and have termed it cam-type impingement.

Arthroscopic or open debridement of the osteoarthritic ankle has been shown to have much less success in alleviating symptoms than in ankles with impingement without OA.6 Most arthritis observed in the ankle is secondary, meaning that it is incited by a traumatic event or other pathology.16, 17 Primary OA of the ankle is likely multifactorial, however, the ankle seems resistant against primary (idiopathic) osteoarthritis by having articular cartilage that is more resilient to sheer stress than articular cartilages found in the hip and knee.16, 18 There is no comprehensive treatment for this condition, although some measures can be taken to alleviate pain and improve joint flexibility.16 This study proposes cam-type impingement of the ankle as a distinct form of bony impingement that may have implications for long-term function of the ankle and as a possible etiology for ankle OA. We hypothesize that this cam-type deformity would frequently accompany ankles anterior bony impingement, posttraumatic arthritis (PTA), or idiopathic osteoarthritis (OA) compared to ankles in the general population. In addition, due to clinical and surgical observations, a correlation between cam-type impingement and varus hindfoot alignment has also been explored.

Materials and Methods

This study was conducted at the University of Iowa Hospitals and Clinics in the Department of Orthopaedics and Rehabilitation. IRB approval was obtained from the university for both the patients for the study and control patients. Subjects for this study consisted of 269 consecutive patients (149 men and 120 women) with preoperative standing lateral ankle radiographs who had surgical diagnoses of: anterior ankle impingement (120 patients), idiopathic osteoarthritis (61), or posttraumatic arthritis (88) from 2004-2009 at the University of Iowa Hospitals and Clinics. The mean age of patients was 45 years (12 to 81). A control group was composed of 41 (35 women and 6 men; ages 16-69, mean 40) consecutive patients from July-August of 2009 with asymptomatic ankles, who had radiographs taken for other conditions. All patients were collected from one of four surgeons to eliminate possible inter-operator discrepancies.

Lateral ankle radiographs were obtained using a standard protocol (55 KV, 2.5 MAS, 45” distance, no grid and small focal spot) with the patients being instructed to place the lateral side of the affected foot against the digital receiver with the malleoli perpendicular to the receiver surface, placing equal weight on each foot. Images were templated and measured using OrthoView software (Meridian Technique Ltd.).

Measurements were made on each lateral ankle radiograph using the following protocols:

Cam ratio- a line was drawn parallel to the superior surface created by the navicular, cuneiform, and metatarsal bones. This line is then translated over the subtalar joint space, just beneath the lateral process of the talus. From this line, measurements are taken on a perpendicular axis to the widest and narrowest aspects of the talus, with the latter generally occurring at the neck. Both measures must be taken anterior to the apex of the talar dome. The cam ratio is the narrow (neck)/ wide (body) calculation (Fig. 3).

Figure 3. Lateral view of the ankle with the standard measure of the CAM ratio.

Figure 3

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α Angle- the α angle was measured by first identifying the largest circle that matches the curvature of the inferior tibial articular surface. This circle was then matched and overlayed with the curvature of the talar dome both medially (anterior dome) and laterally (posterior dome). A positive α sign is generated if the radius of the talar dome exceeds the radius of the circle anteriorly as part of a normal continuation of the talus. To quantify this measurement, a straight vertical line is made to bisect the circle. From the center of the circle, a straight line is drawn to the anterior point at which the radius is exceeded as part of a normal continuation of the talus (non-osteophytic bone and no gutter between). The angle between these two lines is the angle α (Fig. 4). If only one dome is distinguishable, the same measure is recorded for both domes. Measurements larger than 50 degrees (Mean-2SD for all control ankles) are considered negatives (no measurement recorded). The medial and lateral talar dome of the talus is identified by their outline location on the lateral xray. This was verified on 3D CT scan on 5 of the patients to confirm that the medial dome was the prominent and most anterior one on the impingement cases (Fig. 5a,b,c).

Figure 4. Lateral view of the ankle with the standard measure of the alpha angle of the medial and lateral talar domes.

Figure 4

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Figure 5. Lateral view of the ankle demonstrating the cam impingement and the difference between the medial outline of the dome (white arrow) and the lateral (black solid arrow). The medial border extends more anteriorly than the lateral (a). The frontal view for the 3D CT scan (b) and the lateral view (c) demonstrate the difference between the medial border (arrows) and the lateral extend of the talar dome and neck of the talus.

Figure 5

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Calcaneal Pitch (Calcaneal inclination angle) (Fig. 6a) – as described by Thomas et al., the angle created between the supporting surface and a line from the most anterior plantar point of the calcaneal tubercle to the most anterior plantar point of the calcaneus at the calcaneal cuboid joint. 19

Figure 6. Lateral view showing the standard measure of the calcaneal pitch angle (a) and arch height (b).

Figure 6

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Arch Height - arch height was measured from a horizontal line from the inferior-most point of the calcaneus to the inferior bony surface of the great toe sesamoids. The distance from this line to the inferior-most aspect of the medial cuneiform bone is the arch height (Fig. 6b).

Distal Tibial Articular Angle - the distal tibial articular angle was measured from a straight line that bisects the tibia and a straight line that crosses the inferior surfaces of the anterior and posterior tibial plafond. The angle between these two lines is the distal tibial articular angle (Fig. 7).

Figure 7. Lateral view of the standard measure of the distal tibial articular angle.

Figure 7

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These measures were performed by two of the investigators (DN) and (TV). Both individuals received standard instruction on how to make the radiographic measures prior to the study. Both inter- and intraobserver reliability were tested for this study. Tests for both were completed on 15 patients (selected using random number generator) with the recorders blind to one another and to previous results. Measures were taken two additional times for these patients per observer, with 48 hours between studies.

For reliability data, intraclass correlation coefficients were calculated (ICCs) for each measure. A Student's t-test was used to determine significance of all other measuremetns. A p value <0.05 was determined to be statistically significant.

Source of Funding

Funding for this study was provided in part by the Bryan and Nancy Den Hartog Foot and Ankle Sports Medicine Research Fund, through the University of Iowa Foundation.

Results

Intra-observer testing for both investigators for these measures showed high reliability using ratings based on two published reports.20, 21 Inter-observer reliability is shown in Table 1. Each measurement has a good level of agreement, with arch height and calcaneal pitch scoring as the most reliable.20 Measurement of angle α was the least reliable numerically, but raters were unanimous in identifying ankles as positive or negative. Average differences between measurers were as follows: medial α angle – 8.9 degrees, distal tibial articular angle – 1.01 degrees, calcaneal pitch – 0.0033 degrees, arch height – 0.025 cm, cam ratio – 0.0308.

Table 1.

Inter-observer reliability for the measures described.

Radiographic measure Reliability(ICC)
Medial α angle 0.49098
Lateral α angle 0.55811
Distal tibial Articular Angle 0.65180
Calcaneal pitch 0.86407
Arch Height 0.81201
Cam ratio 0.60221
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In this study, 91 of the 269 (34%) total population (impingement, OA, and PTA ankles) generated a positive α α angle measurement on the medial talar dome, and 47 (17.4%) measured positively on the lateral dome. Medial α angles, if positive, averaged 36.46 degrees (range 15- 50) while lateral α angles averaged 38.62 degrees (range 19-50). Table 2 shows average measurements for ankles with and without positive medial α signs. Among the control ankles, 1 (2.4%) was measured to have a positive α sign on both the medial and lateral talar dome according to the guidelines we created.

Table 2.

Measurements for ankles with (positive) and without (negative) medial α signs. Two-tailed t-tests were performed to generate p values.

Positive (n) Distal Tibial Articular Angle (degrees) Calcaneal Pitch (degrees) Arch Height (cm) Cam Ratio
91 Averages 82.42 21.59 2.521 0.8733
SD 5.013 4.917 0.5549 0.07981
Negative
178 Averages 83.76 19.80 2.264 0.7898
SD 5.256 6.118 0.6244 0.1092
Significance p=0.0455 p=0.0162 p=0.001 p<.0001
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Control ankles carried an average α angle of 65 (64.77 medial, 65.23 lateral) with a standard deviation of 7.616. Averages for distal tibial articular angle, calcaneal pitch, and arch height were 84.15 degrees, 21.76 degrees, and 1.98 cm, respectively. The average cam ratio for control ankles was 0.8638 with a standard deviation of 0.1057.

Patients with differing diagnoses were also compared. Angle α measures, distal tibial articular angle and calcaneal pitch did not differ significantly based on diagnosis. Patients with PTA were found to have a significantly higher arch height (2.53 cm) than those with primary OA (2.20 cm) or impingement (2.30 cm) as well as a significantly higher cam ratio (0.8487 versus OA – 0.8019, impingement – 0.8039). Positive medial α signs were not significantly linked to diagnosis, with each patient pool yielding similar percentages of positive ankles.

A population of 36 patients within this study was selected based on measurements (29 males, 7 females) believed to be prototypical or obvious of cam impingement (medial α angle ≤ 40, cam ratio ≥ 0.875). 10 total patients with PTA (11% of all PTA patients measured), 9 with OA (15%), and 17 with impingement (14%) were included in this group. These patients were analyzed with the rest of the population to determine whether cam impingement may be correlated with cavus foot position. The results are shown in Table 3. None of the patients from the control group qualified for these values. The group of control patients was also compared to these patients to demonstrate the difference of cam ratios found between patients with the prototype cam deformity and those with no ankle complaints (Table 4).

Table 3.

Measurements for ankles with medial α angles smaller than 40 degrees and cam ratios greater than 0.875 degrees compared to those without. Two-tailed t-tests were performed to generate p values.

Med. α≤40, cam ratio≥0.875 (n) Distal Tibial Articular Angle (degrees) Calcaneal Pitch (degrees) Arch Height (cm)
36 Averages 80.97 22.42 2.656
SD 4.837 4.959 0.6313
Other ankles
233 Averages 83.67 20.09 2.304
SD 5.176 5.860 0.5978
Significance p=0.0036 p=0.0245 p=0.0012
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Table 4.

Measurements for ankles with (positive) medial α signs of 40 degrees or less and cam ratio of 0.875 or larger from pathologic patients compared to those with no ankle complaints (control). Two-tailed t-tests were performed to generate p values.

Medial α ≤ 40, cam ratio ≥ .875 (n) Cam ratio
36 Average 0.9362
SD 0.0342
Control
41 Average 0.8638
SD 0.1057
Significance p=0.0002
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Discussion

This study identifies a percentage of patients with anterior ankle complaints who demonstrate a specific type of talar bony morphometry. These patients appear to have either flattened talar domes which extrude into the anterior gutter space, reducing the amount of curved articular surface with which the tibia can articulate, or a thicker anterior neck just distal to the talar body articular surface creating the cam impingement effect. Standing lateral radiographs were investigated as a means for identifying patients with this particular anatomy. The most significant deformities were defined as patients with medial α measure less than 40 degrees and a cam ratio of .875 or higher (13% of all patients with PTA, OA, or anterior impingement). Images 5 and 6 depict ankles with this anatomy.

Prototypical cam-type ankles were found to have a significantly higher cam ratio than those with no α sign. The results were similar when these patients were compared to a control group with no ankle complaints. These control ankles had a very low incidence of a positive α sign and elevated cam ratio. This finding demonstrates a correlation between extraneous anterior talar bone presence and a decreased head/neck discrepancy in the talus. Patients from each diagnosis were equally represented among patients who generated a positive αsign on the medial dome as well as among the patients with the highest degree of cam-type deformity. These results indicate that this anatomy is not isolated to certain patient pools and that it may play a role in the development of osteoarthritis, as indicated previously.

Age, obesity, gender, type of fracture and procedure performed have all previously been identified as risk factors for the development of PTA.22 Between 70-75% of ankle arthritis is posttraumatic in origin and the average latency time between injury and development of arthritis has been shown to be around 20 years.22 Treatment for this condition is both difficult and controversial, with some studies suggesting total ankle replacement (TAR) and others more conservative measures.23 Elimination of an abnormal talar process like the one described in this study may be beneficial in treating patients with ankle impingement, may reduce the treatment failure rate, and may also reduce a patient's risk for degenerative arthritis. Another hypothesis proposed by this study is that abnormal talar morphology can lead to increased pathologic contact between the tibia and talus, increasing the chances of fracture and later PTA. Similar risk factors exist for the development of idiopathic OA, with age being the strongest risk factor.24 One study has shown that any abnormal joint mechanics can have long-term detrimental effects, specifically leading to an increase in arthritis.25 By this logic, cam-type impingement of the ankle may be related to development of idiopathic OA. It is unclear if this process is reversible, but restoration of normal joint mechanics can often allow the joint to heal.25

Previous research has shown that the distal tibial articular angle in relation to the tibial axis is related to anterior articular stress in the ankle.26 Ankles with a positive medial α sign had a significantly different distal tibial articular angle, and those with the most serious cam-type tali (med. α≤40, cam ratio≥0.875) had a much smaller anterior angle than those without. This position may play a role in the severity of cam impingement, as it is likely that these patients would have more pathologic contact at the tibio-talar joint than those with larger angles.

The data in this study also indicate a strong correlation between cam-type deformity and pes cavus. Thomas et al. found an average calcaneal pitch among patients over the age of 31 with no history of ankle complaints to be 20.6 degrees, with males and females carrying the same average.19 These results are supported by the control ankles in this study, which carried an average pitch of 21 degrees. This study suggests that these results may be typical of ankles with a history of pathology as well, which averaged 20.4 degrees in our measures. However, ankles with a positive medial α sign were significantly higher-arched than those without (Table 2). Furthermore, patients with a prototypical cam lesion (med. α≤40, cam ratio≥.875) were found to have a more cavus foot compared to those without, with an average pitch 2 degrees larger and arch 3 mm higher than all pathological ankles measured. This correlation may be causative, as increased arch height leads to external rotation of the tibia and reduced joint space within the tibio-talar junction.27, 28, 29 Due to the limitations of only viewing standing lateral radiographs for this study, it is also possible that these patients, who were experiencing some ankle pathology at the time of image collection, were under-loading their affected foot during the procedure, causing an increase in arch height to be perceived laterally.

The reliability results for this study indicate that for each individual, a high level of consistency can be expected when making these measurements. Inter-rater reliability is slightly lower when measuring α angles and cam ratios. This is likely due to the slightly subjective nature of determining these measurements and experience in reading radiographic images. These measures are all considered to have a good level of inter-rater reliability according to an objective metric.20 Based on average differences, we can expect to calculate α angles within 10 degrees of one another and cam ratios within 0.03 (unitless) reliably with this method. This finding demonstrates that the cam ratio may be a more reliable means of measuring the degree of cam impingement in these patients. Ultimately, the cam ratio may be a better clinical tool as a measure of cam impingement when compared to the α angle, based both on reliability of measurements between observers and the ease of completing this metric.

This study is limited in its scope in several ways. Primarily, the retrospective nature of this study leaves many variables uncontrollable. Secondly, the use of standing lateral radiographs exclusively limits the total understanding of ankle morphology that may be present. Lateral ankle radiographs are also subject to rotational effects, although attempts were made to eliminate any possible rotational alterations by using ratios and angles for the majority of measurements.26, 30 The use of a standard protocol for collection of the images also served to eliminate this bias. A similar study using 3D CT reconstructions would be more useful in determining the entire bony anatomy present with this deformity. Further follow-up should include prospective research following patients with these measurements to determine surgical outcomes, clinical presentations, and demographic trends. A pediatric study of this anatomy would be useful to confirm its developmental nature. It is also unknown whether more significant measuremetns of the angle α and cam ratio lead to more severe clinical outcomes.

Footnotes

Disclosures: Ned Amendola, MD - royalties (Arthrex, Inc.); stock options (Arthrosurface)

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