Correlation Between Postoperative Morphologic Changes of Ankle Mortise and Ankle Joint Function After Pilon Fracture

The ankle joint is the largest hinge joint of the human body, which consists of the lower end of the tibiofibula and the trochlea of talus. The inner and outer ankle joint surfaces form ankle mortise. The morphologic changes of ankle mortise can reduce the contact area of the tibiotalar joint, withstand pressure imbalance, and result in load conduction disorder and eventually traumatic arthritis.^1,2 The main purpose of surgery for a pilon fracture is to obtain a stable, painless, and active joint. The surgery should restore the smoothness of the tibiotalar joint surface as much as possible.³ During treatment, attention should be paid to prevent skin necrosis, a blood supply disorder of the limb end, and traumatic arthritis.⁴ Whether the ankle mortise contour reconstructed after pilon fracture is complete and whether the articular surface is smooth not only affect the ankle joint activity but also work as important indices for predicting the occurrence of traumatic arthritis.⁵

The aim of this study was to explore the relationship between the morphologic changes and functional scores of ankle joint on the injured side after a Ruedi-Allgower type III pilon fracture and to analyze whether the morphologic changes of ankle mortise affected ankle joint function and the underlying cause.

Materials and Methods

Results

Indices of ankle mortise on the injured and healthy sides

The difference was not statistically significant in the ankle mortise height between the affected side and the uninjured side (P > 0.05), indicating that the ankle mortise height of the affected side recovered better after surgery, and the difference was not obvious compared with normal ankle mortise. The differences in the ankle mortise width, depth, and coronal and sagittal angles were statistically significant between the affected side and the uninjured side (P < 0.05), indicating that the postoperative ankle mortise width, depth, and coronal and sagittal angles have changed. There was a certain correlation between the change and the function of ankle joint on the affected side (Table 1).

Table 1 Indices of ankle mortise on injured and healthy sides

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Differences between indices of ankle mortise on the injured and healthy sides of excellent, good, and mediocre + poor groups

There was no significant difference in the height difference among the 3 groups (P > 0.05; Table 2), suggesting that the recovery of ankle joint function was not highly correlated with the height of ankle mortise. The difference in the width of ankle mortise was statistically significant among the 3 groups (P < 0.05; Table 2). The q test used for pairwise comparison showed that the differences between width were statistically significant (P < 0.05; Table 3), where the difference in the width of the mediocre + poor group was >2 mm on average and that of the excellent and good groups was <1 mm. Therefore, the width of ankle mortise during surgery had a significant effects on postoperative outcomes. In addition, the differences in the ankle mortise depth and coronal and sagittal angles were statistically significant among the 3 groups (P < 0.05; Table 2). The pairwise q test revealed that there were statistically significant differences among the 3 groups in intraoperative ankle mortise depth and coronal and sagittal angles (P < 0.05; Table 3), suggesting all these factors had significant impact on postoperative outcomes.

Table 2 Differences between indices of ankle mortise on injured and healthy sides of excellent, good, and mediocre + poor groups

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Table 3 Pairwise q test results and P values

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Discussion

The maintenance of the height of ankle mortise depends on the integrity of fibula and the recovery of the top of tibia fornix. The reasons for the change include fibula or lateral malleolus comminuted fracture displacement causing fibula shortening and external rotation; distal tibia comminuted compression fracture not retracted during reduction; and insufficient or no bone grafting at metaphysis.^7,8 In this study, the height change of ankle mortise was small, with a range of <0.5 mm compared with the uninjured side. No statistically significant differences were found in the height change and the ankle joint function score. Probably, the nonobvious loss of ankle mortise height was related to the anatomical reduction of the intraoperative fibula and the emphasis on bone graft filling of metaphysis.

An increase in the width of ankle mortise causes the astragalus to move outward in ankle mortise, resulting in instability of the lateral side of ankle joint, loss of anastomosis on the articular surface, reduction of the weight-bearing area, and uneven pressure, which aggravates ischemia and cartilage degeneration.⁹ Moreover, after the widening of ankle mortise, its restrictive effect on the astragalus is weakened, which results in anterior and posterior instability, eventually leading to ankle joint traumatic arthritis.¹⁰ The reasons for the change in width of ankle mortise include reduction of malleolus medialis fracture, less than ideal displacement of the fibula or lateral malleolus fracture; no reduction or fixation performed on the tibiofibular syndesmosis separation; and a compressed burst fracture at the lower end of tibia end that was not retracted during transverse separation and reduction.^11,12 Herein, the 2-year follow-up results showed that 7 patients in the mediocre + poor group had Mazur scores of the ankle joint of <80, poor function of the ankle joint, and an increase in the width of ankle mortise by >2 mm on the uninjured side, which indicates that the width changes of ankle mortise has a greater impact on the function of the ankle joint in the future.

The depth of ankle mortise increases, causing the astragalus to sink deep inside, and the front is prone to collide and affect joint activity.¹³ Hak et al confirmed by biomechanical experiments that the posterior malleolar fracture can cause the stress center of the ankle joint contact to move forward and inward, so that it withstands huge contact stress during exercises, causing traumatic arthritis of the ankle joint.¹⁴ Zhang et al found that when the distal tibia fracture block was greater than or equal to 10% of the articular surface of the tibial fracture, surgical reduction and fixation of the posterior fracture block were required.¹⁵ The reasons for the depth change of ankle mortise include rupture fracture at the lower end of tibia causing the fracture block to be separated at the sagittal plane and not dissected and fixed during treatment; and displacement of the posterior bone block of distal tibia being difficult to expose during surgery, causing dissatisfactory fixation.¹⁶ For the patients with mediocre + poor scores of ankle joint function, the depth difference of ankle mortise between affected and uninjured sides was 2–5 mm. The 1/2 osteotomy at the lower end of tibia is equivalent to 65% of the normal contact area of ankle joint.

The causes for the angle change of ankle mortise include poor restoration of anterior lateral bone mass (Cha-put nodule) at the lower end of tibia; uneven tibia bone graft filling; improper selection of a steel plate and uneven support; and premature load of the ankle joint.¹⁷ Anatomically, the ankle mortise angle change reflects the change in the load force line of the ankle joint.¹⁸ In this study, the coronal angle of the ankle mortise on the uninjured side was 6.04° ± 1.17°, and the sagittal angle was 11.28° ± 2.81°, which is the load force line of the normal ankle joint. When the distal tibia is damaged by high-energy injury, the accompanying fibula often has a fracture shift, and the perpendicular line of the connection line of the two fornix tops of ankle mortise no longer maintains an approximately right angle with the connection line between the anterior and posterior ankle margins, thus exhibiting the changes of coronal and sagittal angles of ankle mortise. After the deviation of the load force line of ankle joint, the stress distribution of the ankle joint may be changed, resulting in a disorder of the bone joint load and causing arthritis in ankle joint. This study showed statistically significant differences in the ankle mortise coronal and sagittal angles among the 3 groups. In our clinical follow-up, 2 patients with ankle joint Mazur scores of <87 points after external fixation treatment had traumatic arthritis, and the difference in the ankle mortise coronal angle was >5° between the affected and the uninjured sides and that of the sagittal angle was >3°, indicating that the change in ankle mortise angle affects its function.

When reconstructing the ankle mortise anatomy, the length of fibula or lateral malleolus should be restored first, and the fracture end should be initially stabilized; then, with the 3 major fracture blocks (i.e., the malleolus medialis fracture block, the anterior lateral fracture block [Tillaux-Cha-put], and the labium posterius fracture block [Volkmanns triangle]) as the reference points, the tibiocalar articular surface received anatomical reduction.¹⁹ The main points of surgical reconstruction of ankle mortise include restoring the load force line and length of tibia and fibula (especially tibia), that is, restoring the height of ankle mortise, and making the ankle mortise coronal and sagittal angles normal; restoring the anatomical structure of the tibia articular surface and ensuring the surface to be smooth and flat; restoring the anatomical relationship of tibiofibular syndesmosis; preventing the expansion of ankle mortise; and trying to achieve strong and stable fixation, which is good for early activities and prevents morphologic deformation of posterior ankle mortise.^20,21 According to the fracture situation, we should choose suitable inner and outer side panels or judge whether the front and rear are additionally fixed. Attention should be paid to the bone grafting of the metaphyseal bone defect to ensure that the ankle mortise is stable on the top, the reconstructed articular surface is smooth and intact, and the ankle mortise angle is normal.

In summary, anatomical reduction for the width, depth, and coronal and sagittal angles of ankle mortise of patients with Ruedi-Allgower type III pilon fractures exerts significant positive effects on ankle joint function. Thus, the anatomical form of ankle mortise should be restored as much as possible during surgery.