Achilles tendon rupture and medial malleolar fracture are often observed independently in routine medical practice, but their concurrent occurrence is rare. To our knowledge, there have been only 6 case reports of concomitant Achilles tendon rupture and medial malleolar facture in the English literature ^{(Table 1) 3-8. The causes of these concomitant injuries were a motor vehicle accident in 1 case, sport accidents in 2, and a fall or false step on stairs or a ladder in 3.}

Concomitant Achilles tendon rupture and medial malleolar facture due to alpine skiing accidents have been described in some reports ^9-12. Moritz⁹ and Clayton¹⁰ describe such concomitant injuries caused by skiing accidents, but the number or frequency of cases is not indicated. Lugger et al. ¹¹ reported in 1977 that Achilles tendon rupture caused by an alpine skiing accident was complicated by medial malleolar fracture in 10 (5.3%) of 189 cases, and this ratio had not changed in the past 15 years. Obrist et al.¹² reported in 1989 that, out of 408 cases of Achilles tendon rupture caused by alpine skiing accidents, 41 cases (10.0%) also exhibited osteopathy and 37 cases (9.1%), medial malleolar fracture. Based on the above 2 reports, approximately 5 - 9% of cases of Achilles tendon rupture caused by alpine skiing accidents were complicated by medial malleolar fracture. To our knowledge, only one case of such concomitant injuries caused by an alpine skiing accident has since been reported in Japan (oral presentation). Since the incidence of injury to the foot including the ankle joint has generally decreased as ski boots and bindings are developed, improved and spread,^{10, 13, 14} the incidences of these concomitant injuries may have subsequently decreased. Furthermore, the improvement of alpine ski devices might have contributed specifically to the prevention of these injuries.

Obrist et al.¹² report that, of 222 cases of Achilles tendon rupture with causes other than alpine skiing accidents, 5 (2.3 %) were complicated by medial malleolar fracture (causes were non-skiing sport accidents in 3 cases and non-sport accidents in 2), in total, including the 37 cases caused by alpine skiing accidents, 42 (6.6 %) of 630 cases of Achilles tendon rupture were complicated by medial malleolar fracture.

Based on the above, the frequency of Achilles tendon rupture complicated by medial malleolar fracture is estimated to be approximately 2 - 9%, showing that these concomitant injuries are relatively rare.

Figure 4

The mechanism of these concomitant injuries is inconsistent among previous reports: Achilles tendon rupture precedes medial malleolar fracture in some cases, and vice versa in others. Moreover, the presumed cause of medial malleolar fracture is eversion in some cases, and inversion in others.

Lauge-Hansen^{15, 16} classified malleolar fractures based on the injury mechanism in an experiment using amputated legs. Medial malleolar fracture corresponds to one of the following 3 types of injury: stage I of a pronation-external rotation (PER) injury, stage I of a pronation-abduction (PA) injury, and stage I of a pronation-dorsiflexion (PD) injury. Among 6 cases in the previous reports which refer to the Lauge-Hansen classification, 5 cases correspond to PER or PA injury, and 1 to PD injury. Since solitary medial malleolar fracture does not occur in supination-external rotation (SER) injury or supination-adduction (SA) injury, medial malleolar fracture in these cases could not have been caused by inversion.

Since contraction of the triceps surae muscle creates inversion motion of the subtalar joint with ankle flexion, the pronated position or eversion motion of the foot causes hypertension of the Achilles tendon ¹⁷. Therefore, medial malleolar fracture and Achilles tendon rupture might occur concomitantly in the pronated position or during hyper-eversion motion of the foot. The foot should always be in the pronated position in alpine skiing, which may be a reason why relatively many investigators have reported these concomitant injuries in alpine skiing. On the contrary, the tension of the Achilles tendon may be relieved in the supinated position or during inversion motion of the foot. Therefore, lateral malleolar fracture and Achilles tendon rupture could not occur concomitantly.

Complication of Achilles tendon rupture suggests the presence of axial loading as the external force causing injury, but axial load is not taken into consideration in the experiment performed by Lauge-Hansen. Funk et al.¹⁸ investigated the relationship between the axial loading noted in traffic injuries and malleolar fracture in cadavers, and showed that axial loading increases the compatibility of the talocrural joint and its stability against the external rotatory force. Thus, external rotatory force loaded with axial impact reaches a maximal value at a small rotation angle, and causes malleolar fracture.

Based on the above, concomitant Achilles tendon rupture and medial malleolar facture are considered to be classified into the following 2 types based on their injury mechanisms: 1) Axial impact and external rotation type injury (Stage I PER injury in the presence of axial impact), and 2) Excessive dorsal flexion type injury (Stage I PD injury). Hyper-eversion motion, axial impact or excessive dorsal flexion may cause strong contraction of the triceps surae muscle which may result in Achilles tendon rupture. Therefore, axial impact and external rotation type injury may occur when external rotation or hyper-eversion force is loaded in the pronated position in the presence of axial impact on the middle sole which is insufficient to cause plafond fracture. Excessive dorsal flexion type injury may occur when strong dorsal flexion force is loaded on the sole of the forefoot in the pronated position.

Based on the conditions at the time of injury, our case might be caused by the type 1 injury mechanism of “Axial impact and external rotation type injury”. In the pronated position, reflex contraction of the triceps surae muscle might occur and rupture the Achilles tendon on landing, and an external rotatory force with axial load might be applied to the foot and fracture the ankle joint’s medial malleolus.

Obrist et al.¹² report that Achilles tendon rupture was not accurately diagnosed at first examination in 2 of 42 cases of these concomitant injuries. In at least 3 reported cases, medial malleolar fracture or Achilles tendon rupture was not accurately diagnosed at the first examination.^{5, 7, 8} In these reports, Achilles tendon rupture was diagnosed based on the physical findings, but medial malleolar fracture was overlooked because only the lateral view was acquired on plain X-ray examination. In contrast, at an emergency outpatient clinic, medial malleolar fracture was diagnosed by plain X-ray radiography without detailed physical findings and Achilles tendon rupture was overlooked. It is therefore important to consider concurrent occurrence of Achilles tendon rupture and medial malleolar fracture at first examination, and to obtain both detailed physical findings and accurate imaging findings.

The patient was informed that data from the case would be submitted for publication, and gave his consent.