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.
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.
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 3–4). 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 Ruptures
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.
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.
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.
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.
- Garras DN, Raikin SM, Bhat SB, Taweel N, Karanjia H. MRI is unnecessary for diagnosing acute Achilles tendon ruptures: clinical diagnostic criteria. Clin Orthop Relat Res. 2012;470(8):2268–2273. https://doi.org/10.1007/s11999-012-2355-y PMID: doi:10.1007/s11999-012-2355-y [CrossRef]22538958
- Thevendran G, Sarraf KM, Patel NK, Sadri A, Rosenfeld P. The ruptured Achilles tendon: a current overview from biology of rupture to treatment. Musculoskelet Surg. 2013;97(1):9–20. https://doi.org/10.1007/s12306-013-0251-6 PMID: doi:10.1007/s12306-013-0251-6 [CrossRef]23546858
- Renninger CH, Kuhn K, Fellars T, Youngblood S, Bellamy J. Operative and non-operative management of Achilles tendon ruptures in active duty military population. Foot Ankle Int. 2016;37(3):269–273. https://doi.org/10.1177/1071100715615322 PMID: doi:10.1177/1071100715615322 [CrossRef]
- Combalia A, Nardi J. Roentgenographic assessment of Achilles tendon rupture. J Accid Emerg Med. 1995;12(3):230–231. https://doi.org/10.1136/emj.12.3.230 PMID: doi:10.1136/emj.12.3.230 [CrossRef]8581262
- Wiegerinck JI, Somford MP, Hoornenborg D, van Dijk CN. Eponyms of the Kager triangle. J Bone Joint Surg Am. 2012;94(10):e67. https://doi.org/10.2106/JBJS.K.01074 PMID: doi:10.2106/JBJS.K.01074 [CrossRef]22617933
- Tonarelli JM, Mabry LM, Ross MD. Diagnostic imaging of an Achilles tendon rupture. J Orthop Sports Phys Ther. 2011;41(11):904–904. https://doi.org/10.2519/jospt.2011.0422 PMID: doi:10.2519/jospt.2011.0422 [CrossRef]22048317
- Cetti R, Andersen I. Roentgenographic diagnoses of ruptured Achilles tendons. Clin Orthop Relat Res. 1993;286:215–221. https://doi.org/10.1097/00003086-199301000-00032
- Maffulli N. The clinical diagnosis of subcutaneous tear of the Achilles tendon: a prospective study in 174 patients. Am J Sports Med. 1998;26(2):266–270. https://doi.org/10.1177/03635465980260021801 PMID: doi:10.1177/03635465980260021801 [CrossRef]9548122
- Chiodo CP, Glazebrook M, Bluman EM, et al. American Academy of Orthopaedic Surgeons. American Academy of Orthopaedic Surgeons clinical practice guideline on treatment of Achilles tendon rupture. J Bone Joint Surg Am. 2010;92(14):2466–2468. PMID:20962199
- Kou J. AAOS Clinical practice guideline: acute Achilles tendon rupture. J Am Acad Orthop Surg. 2010;18(8):511–513. https://doi.org/10.5435/00124635-201008000-00008 PMID: doi:10.5435/00124635-201008000-00008 [CrossRef]20675644
- Pingel J, Petersen MCH, Fredberg U, et al. Inflammatory and metabolic alterations of Kager's fat pad in chronic Achilles tendinopathy. PLoS One. 2015;10(5):e0127811. https://doi.org/10.1371/journal.pone.0127811 PMID: doi:10.1371/journal.pone.0127811 [CrossRef]25996876
- Theobald P, Bydder G, Dent C, Nokes L, Pugh N, Benjamin M. The functional anatomy of Kager's fat pad in relation to retrocalcaneal problems and other hindfoot disorders. J Anat. 2006;208(1):91–97. https://doi.org/10.1111/j.1469-7580.2006.00510.x PMID: doi:10.1111/j.1469-7580.2006.00510.x [CrossRef]16420382
- Ly JQ, Bui-Mansfield LT. Anatomy of and abnormalities associated with Kager's fat pad. AJR Am J Roentgenol. 2004;182(1):147–154. https://doi.org/10.2214/ajr.182.1.1820147 PMID: doi:10.2214/ajr.182.1.1820147 [CrossRef]
- Assal M, Stern R, Peter R. Fracture of the ankle associated with rupture of the Achilles tendon: case report and review of the literature. J Orthop Trauma. 2002;16(5):358–361. https://doi.org/10.1097/00005131-200205000-00013 PMID: doi:10.1097/00005131-200205000-00013 [CrossRef]11972082
Mean Sensitivity and Specificity of Lateral Radiographs for the Diagnosis of Achilles Tendon Rupturesa
|Normal ankle, No.||50|
|Rupture of Achilles tendon, No.||50|