Orthopedics

Feature Article 

The Reliability of Kager's Triangle in Detecting Acute Achilles Tendon Ruptures

Christopher M. Belyea, MD, MBA; Kevin P. Krul, MD; Gregory Lause, MD, MS; Anthony Magee, MD; Claude D. Anderson, MD; Paul M. Ryan, MD

Abstract

The Achilles tendon is a common site of anatomical injury among athletes and those participating in recreational sporting activities. Acute Achilles tendon ruptures are often misdiagnosed as a sprained ankle and are more common in the male population. Mechanism of injury is often a noncontact injury, resulting from sudden forced plantar flexion or violent dorsiflexion in a plantar flexed foot. Delays in diagnosis and treatment may complicate the clinical outcome. Because findings may be subtle on history and physical examination, the use of readily available adjunctive studies is important for practitioners. One method is the analysis of Kager's triangle on lateral ankle radiographs. Obscuration of Kager's triangle has been described as a radiographic indicator of Achilles tendon rupture, but the sensitivity and specificity of this finding have been poorly reported. [Orthopedics. 2020; 43(2): e91–e94.]

Abstract

The Achilles tendon is a common site of anatomical injury among athletes and those participating in recreational sporting activities. Acute Achilles tendon ruptures are often misdiagnosed as a sprained ankle and are more common in the male population. Mechanism of injury is often a noncontact injury, resulting from sudden forced plantar flexion or violent dorsiflexion in a plantar flexed foot. Delays in diagnosis and treatment may complicate the clinical outcome. Because findings may be subtle on history and physical examination, the use of readily available adjunctive studies is important for practitioners. One method is the analysis of Kager's triangle on lateral ankle radiographs. Obscuration of Kager's triangle has been described as a radiographic indicator of Achilles tendon rupture, but the sensitivity and specificity of this finding have been poorly reported. [Orthopedics. 2020; 43(2): e91–e94.]

The Achilles tendon is the strongest tendon in the human body. However, it remains a common site of anatomical injury among athletes and those participating in recreational sporting activities. The annual incidence of acute Achilles tendon ruptures is estimated to be 18 cases per 100,000 individuals.1 It has been reported that 44% to 83% of Achilles tendon ruptures occur during sporting activities.2 Studies have shown that spontaneous rupture of the Achilles tendon is common in the active duty military population.3 Delay in treatment, whether operative or nonoperative, reportedly has detrimental effects on the final outcome.1 Prompt diagnosis of Achilles tendon ruptures is important for timely surgical or nonoperative therapy, with the goal of patients being able to return to sports or full active duty.

Acute Achilles tendon ruptures are often identified by findings on physical examination. However, 20% of these ruptures may be missed by primary care physicians and up to 25% are missed or primarily misdiagnosed in the emergency department.4 Therefore, ankle radiographs could be a valuable supplement to diagnosis.

Kager's triangle, also known as the pre-Achilles fat pad, is both an anatomical structure and a landmark seen on lateral radiographs of the ankle (Figure 1). It is a lipomatous area demarcated by the flexor hallucis muscle and tendon anteriorly, the superior cortex of the calcaneus inferiorly, and the Achilles tendon posteriorly.5 The fat pad may be disrupted or distorted by effusion and soft tissue changes resulting from an Achilles tendon rupture6 (Figure 2).

Normal lateral ankle radiographs showing intact Kager's triangle. The radiograph on the right outlines Kager's triangle with a red triangle.

Figure 1:

Normal lateral ankle radiographs showing intact Kager's triangle. The radiograph on the right outlines Kager's triangle with a red triangle.

Lateral ankle radiograph showing obliteration of Kager's triangle secondary to Achilles tendon rupture.

Figure 2:

Lateral ankle radiograph showing obliteration of Kager's triangle secondary to Achilles tendon rupture.

There is a paucity of literature regarding the sensitivity and specificity of disruption of Kager's triangle on lateral radiographs in the setting of acute Achilles tendon ruptures. The aim of this study was to report the sensitivity and specificity of this radiographic finding to better determine its diagnostic utility.

Materials and Methods

Appropriate approval was obtained from the Quality Improvement Committee at the authors' institution to conduct this study. The authors identified a cohort of 50 consecutive isolated acute Achilles tendon ruptures with plain lateral radiographs of the ankle among patients who presented to their institution. All of the patients who were reviewed had undergone open Achilles tendon repair; this ensured that there was an Achilles tendon rupture. For comparison, a control cohort was created using presurgical lateral ankle radiographs of 50 consecutive patients who underwent soft tissue operations without Achilles tendon pathology during the same period. There were only soft tissue procedures, no fractures, in the control cohort. All radiographs included in the study involved acute soft tissue injuries without any prior pathology. The radiographs in both cohorts were de-identified and combined in a randomized order.

All 100 radiographs were placed into PowerPoint (Microsoft, Redmond, Washington) and then analyzed by 2 fellowship-trained foot and ankle orthopedic surgeons (C.D.A., P.M.R.) who were looking for disruption of Kager's triangle only. The surgeons were blinded to the identity, diagnosis, and medical history relating to each radiograph. Each radiograph was interpreted and documented independently. The surgeons were blinded to each other's results.

Results

The combined diagnostic results for the 100-patient randomized cohort revealed that the study's sensitivity for using Kager's triangle to detect an Achilles tendon tear was 87%. The specificity was calculated to be 81% (Table 1, Figures 34). The 2 blinded surgeons agreed on the presence or absence of Kager's triangle 80% of the time for an interobserver reliability kappa coefficient of 0.76.

Mean Sensitivity and Specificity of Lateral Radiographs for the Diagnosis of Achilles Tendon Rupturesa

Table 1:

Mean Sensitivity and Specificity of Lateral Radiographs for the Diagnosis of Achilles Tendon Ruptures

Pie chart depicting mean true-positive and false-negative results of lateral radiographs for diagnosing Achilles tendon ruptures.

Figure 3:

Pie chart depicting mean true-positive and false-negative results of lateral radiographs for diagnosing Achilles tendon ruptures.

Pie chart depicting mean true-negative and false-positive results of lateral radiographs for diagnosing Achilles tendon ruptures.

Figure 4:

Pie chart depicting mean true-negative and false-positive results of lateral radiographs for diagnosing Achilles tendon ruptures.

Discussion

Early and reliable diagnosis of acute Achilles tendon ruptures is emphasized, as a delay in treatment can reduce the chances of patients returning to their preinjury level of athletic activity.4,7

The diagnosis of Achilles tendon ruptures is largely based on well-described findings on physical examination. The Thompson test—lack of passive plantar flexion with calf squeeze while the patient is in the prone position—has a sensitivity and a specificity of 0.96 and 0.93, respectively.8 The Matles test is also performed with the patient in the prone position with bilateral knees flexed to 90°. An asymmetry in the resting ankle position with the affected side having increased dorsiflexion of 20° to 30° has a sensitivity and a specificity of 0.88 and 0.85, respectively.8 Finally, a palpable gap in the tendon has a sensitivity of 0.73 and a specificity of 0.89.8 The American Academy of Orthopaedic Surgeons practice guideline indicates that the presence of 2 or more of these abnormal findings on physical examination is diagnostic of an acute Achilles tendon rupture.9,10 Despite the relatively high specificity of these tests, acute injures are missed on index examination in the emergency department and primary care office.

Kager's triangle may stabilize surrounding ankle structures and motion about the ankle joint.11,12 The utility of disruption or obliteration of Kager's triangle on lateral ankle radiographs has been reported only once previously.7 Those authors reported that the sensitivity of Kager's triangle was 100%. However, that study was limited in its design, as the individuals reviewing the radiographs were not blinded to the diagnosis.7 Other studies have examined other lateral radiographic parameters, including Toygar's angle (Figure 5), the abnormal decrease in the angle of the posterior skin surface shadow, and Arner's sign (Figure 6), an abnormal anterior deviation of the Achilles tendon. Prior studies have reported Toygar's angle and Arner's sign to have sensitivities ranging from 12% to 78%.7,13

Lateral radiograph showing an abnormal decrease in the angle of the posterior skin surface shadow, known as Toygar's angle.

Figure 5:

Lateral radiograph showing an abnormal decrease in the angle of the posterior skin surface shadow, known as Toygar's angle.

Lateral radiograph showing anterior deviation of the Achilles tendon, a phenomenon also known as Arner's sign.

Figure 6:

Lateral radiograph showing anterior deviation of the Achilles tendon, a phenomenon also known as Arner's sign.

The radiographic analysis of Achilles tendon ruptures may also identify some rare concomitant injuries. Several case reports have described an infrequent association of a closed oblique to vertical medial malleolus fracture with complete Achilles tendon rupture.14 These rare injuries appear to have an association with skiing injuries.14

This study had limitations. Because both reviewers were orthopedic surgeons, the results may not be generalizable to providers from other specialties. In addition, intraobserver reliability was not tested. Future studies including primary care providers and emergency department providers would be worthwhile.

Conclusion

In this cohort, obscuration of Kager's triangle provided a reliable and simple method of detecting an acute Achilles tendon rupture. The calculated specificity and sensitivity of the blinded review suggested that the obliteration or disruption of Kager's triangle may be a valuable diagnostic tool as an adjunct to physical examination. The ability to use this as a screening tool with difficult physical examinations, for telemedicine in remote areas, or as a reliable substitute for advanced imaging can avoid costly secondary studies or evacuations and improve the detection of tears at the point of care.

References

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Mean Sensitivity and Specificity of Lateral Radiographs for the Diagnosis of Achilles Tendon Rupturesa

ParameterValue
Normal ankle, No.50
Rupture of Achilles tendon, No.50
Sensitivity, %87
Specificity, %81
Kappa coefficient0.76
Authors

The authors are from the Department of Orthopaedic Surgery, Tripler Army Medical Center, Honolulu, Hawaii.

The authors have no relevant financial relationships to disclose.

The views expressed in this manuscript are those of the authors and do not reflect the official policy of the Department of the Army, Department of Defense, or US Government. All authors are employees of the US Government. This work was prepared as part of their official duties and as such, there is no copyright to be transferred.

Correspondence should be addressed to: Christopher M. Belyea, MD, MBA, Department of Orthopaedic Surgery, Tripler Army Medical Center, 1 Jarrett White Rd, Honolulu, HI 96859 ( cmbelyea@gmail.com).

Received: September 04, 2018
Accepted: January 14, 2019
Posted Online: December 16, 2019

10.3928/01477447-20191212-05

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