Ring External Fixation in the Foot and Ankle

Limb Axis Malalignment and Ankle Arthritis

Malalignment and/or shortening of the lower extremity are believed to accelerate degenerative arthritis of the hip, knee, and ankle. Because of the prevalence of knee arthritis, the preponderance of current literature relative to the relationship of extremity malalignment and arthritis relates to the knee. The effectiveness of axis realignment on halting the progression of knee arthritis has not been consistently successful. In a 60-year-old female cohort the 10-year survivorship of high tibial osteotomy was shown to be a dismal 32%. Biochemical and biomechanical properties of ankle joint cartilage differ from knee joint cartilage, suggesting that the ankle joint may have more inherent reparative properties, and axis realignment may have greater utility. It is therefore presumed that correction of tibial axis malalignment for the purposes of ankle joint preservation may merit consideration in some patients. Unfortunately, there is no evidence-based literature that defines specific deformity parameters or angular thresholds that invariably lead to arthritis. Mid- to long-term follow-up studies, however, have demonstrated the effectiveness of osteotomy in relieving ankle pain and delaying joint sacrificing procedures such as fusion or replacement in patients without advanced ankle arthritis. In addition to axis realignment for joint preservation, realignment may also need to be considered even in circumstances of advanced ankle arthritis that necessitate fusion or replacement when the malalignment could lead to undesirable implant loading and/or a non-plantigrade foot.

Ring fixation and gradual correction are the procedure of choice in circumstances of complex multiplanar deformity, multiply operated and/or poor soft tissue, anticipated neurovascular compromise with acute correction, as well as bone loss with limb shortening.

Normal Alignment and Deformity Measurement

A mechanical axis is defined as a line connecting the center points of two joints. In the frontal or coronal plane, normal alignment of the lower extremity is defined as the mechanical axis of the lower extremity , a straight line that extends from the center of the hip joint through the knee to the center of the ankle joint ( Fig. 31-2 ). The mechanical axis inferior to the ankle then extends down through the center of the talus and ends approximately 1 cm medial to the center vertical axis of the calcaneal tuberosity ( Fig. 31-3 ).

Despite variations in body morphology, the weight-bearing mechanical axis is consistent and generally falls 10 mm medial to the center of the knee joint. When the mechanical axis of the lower extremity falls medial or lateral to the expected position in the center of the knee, this is called a mechanical axis deviation (MAD). Significant MADs are associated with symptomatic joint pain and the risk of osteoarthritis from asymmetric joint loading.

Variations of the mechanical axis through the knee joint beyond 1 cm are not found in asymptomatic populations, while variations in the weight-bearing line of the tibia lie within 15 mm of the lowest calcaneal point in 95% of asymptomatic populations.

An anatomic axis is a line connecting two midpoints in the diaphysis of a long bone. In the coronal plane below the knee, the mechanical axis and the anatomic axis of the tibia are parallel. The anatomic axis in the tibia may fall just medial, about 3 mm, to the mechanical axis. Most radiographs to evaluate and follow patients with distal tibial deformity will not include the knee joint, so a proximal anatomic axis reference is typically used ( Fig. 31-4 ).

A joint line is drawn parallel to the articular surface of a joint, and joint angles are measured between a joint line and its intersecting long bone axis. Whether the angle is measured on the medial or lateral aspect (or anterior or posterior aspect) of the intersection must be specified. At the ankle, most deformity correction surgeons measure the superior-lateral intersection between the anatomic tibial axis and the distal tibial joint line (lateral distal tibial angle [LDTA]), while many foot and ankle surgeons historically have measured this angle at the superior-medial intersection (TAS). At the ankle, the normal average LDTA is 89 degrees (normal range, 86–92 degrees). Conceptually, therefore the goal of surgical treatment for a distal tibial coronal plane deformity is to make the distal tibial articular surface perpendicular to the tibial anatomic (or mechanical) axis.

If the deformity of the distal tibia occurs in the metaphysis and above, then the coronal plane deformity is measured by drawing the anatomic axis as the proximal reference line and the patient's LDTA from the uninvolved limb or a standard LDTA of 89 degrees from the center of the ankle joint as the distal reference ( Fig. 31-5 ).

In the coronal plane, if the deformity is located at the ankle joint, it is described as juxtaarticular. This occurs commonly from growth arrest or malunion collapse of a pilon fracture. In these cases, the LDTA is the intersection of the proximal anatomic axis and a line parallel to the distal tibial plafond or talar dome, assuming that they are congruent ( Fig. 31-6 ).

To measure coronal plane alignment distal to the ankle, the hindfoot alignment view radiograph described by Saltzman is used (see Fig. 31-3 ). It is a weight-bearing radiograph that shows the distal tibial, ankle joint, and calcaneal tuberosity on a single view. This radiograph requires a specialized mounting box to angle the radiographic plate 20 degrees from the vertical plane. The long axial view is another radiograph that is non–weight bearing; it visualizes the tibia, subtalar joint, and calcaneal tuberosity and does not require a special mounting box ( Fig. 31-7 ).

In the sagittal plane, the mechanical axis runs from the center of the hip joint to the center of the ankle, passing through the anterior portion of the knee joint. In normal alignment, the anatomic axis of the tibia in the sagittal plane will intersect the anterior fifth of the tibial plateau and bisect the distal tibia at the ankle. The normal distal tibial angle is 80 degrees, measured anteriorly and thus termed the anterior distal tibial angle (ADTA) ( Fig. 31-8 ). The midtibial line intersects the midpoint of the talar dome and then, more distally, usually the lateral talar process, although the exact location varies with the position of ankle dorsiflexion or plantar flexion.

Sagittal plane deformities at or above the metaphysis are measured in a similar fashion to coronal plane deformity, using a proximal anatomic axis reference and a distal ADTA of 80 degrees or the patient's normal ADTA ( Fig. 31-9 ). Juxtaarticular deformities are measured using the proximal anatomic axis line and a line along the distal tibial plafond ( Box 31-1 and Figs. 31-10 and 31-11 ).

The rotational component of a deformity is often the most challenging to measure and treat. Measuring rotation can be performed clinically, radiographically, by fluoroscopy, magnetic resonance imaging (MRI), ultrasound, or computed tomography (CT).

Below the knee joint, a perpendicular line extending from the tibial tubercle normally matches the axis of the second toe. Clinical methods to measure rotation include the thigh-foot angle, the thigh-transmalleolar method, the second toe test, and the footprint method. These measurements are well established, easily performed, and can be used as estimations of rotation. However, they are only approximate because of variations in the position of the patella, the tibial tubercle, and the foot, which result in variability in measurements.

We use the thigh-foot angle most frequently, matching rotation to the contralateral asymptomatic side. If there is significant deformity in the foot, rotation can be determined by palpation of the medial and lateral malleolus at the ankle instead.

Radiographic methods of measuring tibial torsion vary among authors both in technique and reliability, and none have been widely accepted as standard. MRI and ultrasound have also been studied but not widely accepted.

Computed tomography is considered the gold standard for measuring tibial rotation. Lee et al describe the simplest method of measuring tibial torsion by using two-dimensional CT. On a CT scan, tibial rotation is defined as the angle between the transarticular axis of the proximal tibia—a line connecting both posterior condyles and the transmalleolar axis of the ankle—a line connecting the medial and lateral malleoli ( Fig. 31-12 ).

An important factor regarding rotational deformities is their influence on the accuracy of coronal plane angular measurements on standard anteroposterior (AP) radiographs. McCann et al demonstrated, in a sawbone tibia model, that increasing tibial rotation increased error in angular measurements. For example, an internal rotation deformity of 25 degrees led to a false 5-degree increase in the amount of varus measured. Appropriate radiographic techniques to prevent this error are described below.

Standing, full-length, lower-extremity, 51-inch AP radiographs from the hip to the ankle are obtained if there is a known or suspected leg-length discrepancy (LLD) or to assess possible deformity above the knee. A block placed under the shorter limb will assist in accurate measurement and determination of LLD. Deformity in the lumbar spine that creates a fixed pelvic obliquity may also create LLD ( Fig. 31-13 ).

Foot deformity is assessed with weight-bearing radiographs that are accurately obtained perpendicular to the dorsum of the foot for AP views.

The AP foot radiograph is measured for talo–first metatarsal angle, talocalcaneal angle, navicular coverage, and joint subluxation, fusion, coalitions, or arthritis. The lateral foot view is measured for talo–first metatarsal angle, navicular–cuneiform malalignment, talocalcaneal angle, calcaneal pitch, and the joint pathology described for the AP view. Normal foot angles have been well defined by Gentili et al.

Comparison weight-bearing radiographs of the contralateral limb are usually obtained for preoperative planning.

Radiographic Evaluation

Obtaining accurate radiographs is the cornerstone of precise deformity analysis, and the radiographs must be planned to best represent the deformity.

• First, center the beam on the region of the deformity and include the joint proximal and distal to the deformity ( Fig. 31-14 ).

  

• Second, orient the joint toward the beam. If the knee joint is the region of interest, then the patella should be centered within the femoral condyles. The lateral view is at 90 degrees to this. If the distal tibia and ankle are the location of the deformity, then the malleoli should be oriented as if taking an ankle AP radiograph, with the tibial-fibular overlap approximately 6 mm. In equinovarus foot deformity, the radiographer aligns the beam with the foot flat against the floor, angling the leg inward ( Fig. 31-15 ).

  

• Third, obtain standing radiographs whenever possible to assess intraarticular wear or ligamentous laxity contributing to the overall deformity (see Fig. 31-8 ).

• Fourth, if there is a suspected LLD, or if it is unknown whether there is deformity around the knee or hip, then obtain a 51-inch standing AP radiograph and measure the mechanical axis. If the mechanical axis falls outside of the center of the knee joint, then measure the coronal plane joint orientation angles at the hip and knee to determine whether there is a deformity above the distal leg (see Fig. 31-2 ).

Compensatory Deformities

Compensatory deformities can occur in the foot. Deformities of the distal tibia usually are compensated in the subtalar joint and the forefoot. The subtalar joint will compensate for a distal tibial varus deformity by moving into an everted position. Because the average eversion in the subtalar joint is 10 degrees, varus deformities greater than 10 degrees may cause symptoms on the lateral border of the foot. Further compensation occurs in the forefoot, with distal tibial varus compensated by forefoot pronation. This is seen as a valgus forefoot or a plantar flexed first ray when the patient's foot is examined in the non–weight-bearing position. Distal tibial valgus will be compensated with subtalar inversion and forefoot varus ( Fig. 31-16 ).

The need to correct the compensatory deformities depends on their extent and rigidity. The goal of correction is to create a plantigrade foot, and the compensatory deformity may be treated with arthrodesis, osteotomy, or tenotomy to achieve this goal. Muscle imbalance may also be associated with a compensatory deformity and require treatment with tendon transfer ( Table 31-1 ).

Deformity Correction

In the tibia, the point where the proximal anatomic and the distal mechanical axes intersect is termed the apex of the deformity. Mechanically, this apex is the optimal location for an osteotomy to correct the deformity, so it also has been termed the center of rotation of angulation (CORA). If the CORA does not fall at what appears to be the apex of the deformity, then there is a translational component of the deformity or a multifocal deformity.

Once the CORA is determined, then the surgeon must decide the osteotomy location. If possible, the osteotomy is made at the CORA. After correction, the bony segment will be straight, the deformity will be corrected, and the proximal and distal mechanical axis lines will be realigned ( Fig. 31-17 ).

Often, however, deformities in the distal tibia and foot have a CORA that does not permit an osteotomy. This may be due to insufficient distance between the joint and the CORA, as in distal tibial juxtaarticular deformities that have the CORA at or near the joint after distal tibial growth arrest or collapse after ankle pilon fractures. Other reasons include poor soft tissue or sclerotic bone that will not allow good bone healing.

In these cases, the osteotomy must be made proximal to the CORA. Correction of the angular deformity will then require translation at the osteotomy to accurately realign the distal mechanical axis to the proximal anatomic axis.

In the case of a distal tibial varus deformity, the distal tibial segment, along with angular correction, will translate medially to correctly realign the limb mechanical axis ( Fig. 31-18 ). If there is a distal valgus deformity, the distal segment is translated laterally along with the angular correction.

In summary, correction with the osteotomy at the CORA avoids secondary translations. However, it may not be feasible to perform an osteotomy at the CORA for distal tibial and foot deformities. This is an important principle and should be followed with acute or gradual corrections.

Supramalleolar Deformity Classification

A simple method to classify and treat distal tibial and fibular deformity defines four general deformity patterns: (1) tibia only, (2) fibula only, (3) equal tibia and fibula, (4) unequal tibia and fibula. The fourth pattern, when the deformity of the fibula differs from that of the tibia, is seen after distal tibial and fibular fractures. It is usually the most difficult to analyze and to treat. Each pattern may exist with intraarticular deformity secondary to ligamentous laxity or asymmetric cartilage.

Osteotomy Location for Supramalleolar Deformity

A comprehensive deformity analysis must be performed as the first step in planning. Once the deformity pattern and CORA are defined, the location of osteotomies becomes straightforward. An osteotomy at the level of the CORA is the ideal location because only angulation is required for correction. However, as noted, the CORA may be at a suboptimal location for an osteotomy because of multiple factors. These include poor or dense bone quality, damaged soft tissue envelope, insufficient distance from the ankle joint to allow wire placements (three wires are the preferred minimum), and a deformity with significant translation, creating a CORA far away from the site of angular deformity.

Altering the location of the osteotomy from the CORA is frequently necessary, and, as previously discussed, requires translation to obtain correction.

Tibial and Fibular Osteotomy Surgical Techniques

The optimal osteotomy technique preserves periosteal blood supply and minimizes thermal bone necrosis. The use of power saws should be avoided in the tibia. The preferred osteotomy technique varies with the anatomic location. A multiple drill hole technique is preferred in the diaphyseal region of the tibia, whereas a Gigli saw osteotomy is also an option in the metaphyseal regions of the tibia. With gradual correction techniques, the osteotomy is usually completed after an external fixator has been applied. The Gigli saw technique is preferred in less dense bone.