Ring External Fixation in the Foot and Ankle

Douglas Beaman, Richard E. Gellman and Travis J. Kemp

Principles of Deformity Evaluation and Correction

Surgical techniques to correct deformity, whether performed acutely or gradually, require the treating surgeon to understand normal limb alignment and accurate deformity measurement.

Deformity within a bone segment is comprised of length, angulation, rotation, and translation measurements. Angulation and translation are present in both the coronal and sagittal planes, resulting in six measurements that are required to accurately describe a malunion deformity or displaced fracture (Fig. 23-1).

Figure 23-1 Six degrees of freedom. Deformity is described by six measurements: in the coronal plane by (1) angulation of varus or valgus and (2) medial or lateral translation, in the sagittal plane by (3) angulation of procurvatum or recurvatum and (4) anterior or posterior translation, in the axial plane by (5) rotation (internal or external), and (6) length (long or short).

Frontal or coronal plane malalignment is commonly described as varus or valgus, whereas lateral or sagittal plane malalignment is described as either procurvatum or apex anterior and recurvatum or apex posterior. Rotational malalignment is defined as internal or external, referring to the position of the distal leg or ankle in relation to the proximal leg.

Normal Alignment and Deformity Measurement

In the frontal or coronal plane, normal alignment of the lower extremity is defined as the mechanical axis, a straight line that extends from the center of the hip joint through the knee to the center of the ankle joint (Fig. 23-2). 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. 23-3A and B).⁴⁷

Figure 23-2 Mechancial axis. The frontal or coronal plane mechanical axis of the lower extremity. The mechanical axis is the line from the center of the hip joint to the center of the ankle joint. Frontal plane joint orientation angles, average and normal range are shown for the hip, knee, and ankle. avg., average.

Figure 23-3 Hindfoot alignment view radiograph (Saltzman). The anatomic axis of the tibia is extended through the calcaneus, and the midaxial line of the calcaneal tuberosity is drawn. The midline of the calcaneus is usually lateral to the midline of the tibia because of the stepped articulation with the talus at the sustentaculum. The weight-bearing point on the calcaneus normally falls lateral, imparting a valgus moment on the ankle and subtalar joints.

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.⁴⁸ Significant mechanical axis deviations 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,⁴⁷ while variations in the weight-bearing line of the tibia lie within 15 mm of the lowest calcaneal point in 95% of asymptomatic populations.⁵⁶

In the coronal plane below the knee, the mechanical axis and the anatomic axis of the tibia are parallel. An anatomic axis is a line connecting two midpoints in the diaphysis of a long bone. 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. 23-4).

Figure 23-4 Coronal plane alignment. In a tibia with no deformity, an anatomic axis line should fall in the center of the tibial plafond or slightly medial. Next, draw a line parallel to the distal tibial articular surface or talar dome, if they are parallel. The angle described on the lateral side, the lateral distal tibial angle (LDTA) averages 89 degrees in large population studies.

At the ankle, the normal average lateral distal tibial angle (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. 23-5).

Figure 23-5 Coronal plane deformity. An example of an 18-degree varus coronal plane deformity measured as described in Figure 23-4. The proximal anatomic tibial axis and the lateral distal tibial angle are drawn. The lateral distal tibial angle (LDTA) of 90 degrees is normal for this patient, determined from his uninjured side. CORA, center of rotation of angulation.

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. 23-6).

Figure 23-6 Coronal plane periarticular deformity. An example of a 14-degree periarticular varus deformity secondary to late collapse of a pilon fracture. In this case, it is easier to measure the angle formed by the proximal anatomic tibial axis and a line parallel to the talar dome. The CORA, center of rotation of angulation, is detailed under the section Deformity Correction.

To measure coronal plane alignment distal to the ankle, the hindfoot alignment view radiograph described by Saltzman ⁵⁶ is used (see Fig. 23-3A and B). 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. 23-7).

Figure 23-7 Long axial view. The long axial view of the hindfoot is a non–weight-bearing radiograph that is useful in assessing alignment when patients cannot stand on the platform for a Saltzman hindfoot alignment radiograph. It usually demonstrates the subtalar joint, unless there is obstruction by external fixation hardware or a subtalar arthrodesis, as in this case.

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. 23-8). 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.^71,72

Figure 23-8 Sagittal plane alignment. In a tibia with no deformity, an anatomic axis line should fall in the center of the talar dome. Next, draw a line parallel to the distal tibial articular surface. The angle described on the anterior side, the anterior distal tibial angle (ADTA) averages 80 degrees in large population studies.

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. 23-9). Juxtaarticular deformities are measured using the proximal anatomic axis line and a line along the distal tibial plafond (Box 23-1 and Fig. 23-10).

Box 23-1

It is important to understand that malalignment in distal tibial or foot deformities can occur from three causes: malunion in the bone, intraarticular wear in the joint, or ligamentous laxity, creating tilt or asymmetric contact of the joint surface. The visualization of intraarticular wear and ligamentous laxity are best seen on weight-bearing radiographs (Fig. 23-11).

Figure 23-11 Weight-bearing radiograph. The importance of weight bearing to fully assess deformity is demonstrated here. This patient has 12 degrees of varus in the distal tibia from an adolescent growth plate injury and 20 degrees of talar tilt from ligamentous laxity, which creates a total of 32 degrees of varus. A non–weight-bearing radiograph can often miss intraarticular wear and ligamentous laxity. LDTA, lateral distal tibial angle.

Figure 23-9 Sagittal plane deformity. An example of a 10-degree recurvatum sagittal plane deformity measured as described in Figure 23-8. The proximal anatomic tibial axis and the lateral distal tibial angles are drawn. Intersection of the two lines above the apparent apex usually indicates the presence of a translational deformity. ADTA, anterior distal tibial angle; CORA, center of rotation of angulation.

Figure 23-10 Sagittal plane periarticular deformity. An example of an approximately 6-degree periarticular varus deformity secondary to late collapse of a pilon fracture. In this case, it is easier to measure the angle formed by the proximal anatomic tibial axis and a line parallel to the talar dome. A comparison to the uninjured limb is useful with sagittal plane deformities because of variability in the anterior distal tibial angle (ADTA).

The rotational component of a deformity is often the most challenging to measure and treat. Measuring rotation can be performed clinically,22,31,38,64,65 radiographically,^18,24,52 by fluoroscopy,⁷ magnetic resonance imaging (MRI),⁵⁹ ultrasound,^25,26 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,³⁵ and the footprint method.^22,31,64 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.^3,42

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 al³⁵ 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. 23-12).³⁴

Figure 23-12 Computed tomography (CT) scan for tibial rotation measurement. Rotational alignment in the tibia is measured as the angle between a line connecting the posterior condyles of the proximal tibia and a line through the medial and lateral malleoli just above the tibial plafond. A comparison of both tibias is recommended because of significant variability in normal populations.

An important factor regarding rotational deformities is their influence on the accuracy of coronal plane angular measurements on standard anteroposterior radiographs.⁶⁸ McCann et al⁴¹ demonstrated this in a sawbone tibia model. They found that increasing tibial rotation increased error in the angular measurement. 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 anteroposterior (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. 23-13).

Figure 23-13 Fifty-one–inch anteroposterior (AP) standing radiograph. This view allows determination of leg-length discrepancy as well as a mechanical axis check through the center of each knee. This patient has a 3-cm block under the right foot. Individual measurements of femurs and tibias are determined to verify precise location of length discrepancy.

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.¹⁶

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. 23-14).

Figure 23-14 Radiograph: center beam on deformity or joint of interest. The x-ray beam is centered on the ankle or foot but includes enough of the tibial shaft to easily draw an anatomic axis line. The foot is rotated inward or outward, depending on the presence of rotational deformity and the need to visualize either the ankle or the foot.

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.⁴⁶ 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. 23-15).

Figure 23-15 Anteroposterior (AP) foot deformity radiograph. The two radiographs demonstrate the difference between the x-ray beam oriented perpendicular to the floor, termed the “pseudo-AP”, and the beam oriented perpendicular to the dorsum of the foot for a true AP radiograph. The clinical photograph shows the foot appearance of the radiographs.

Third, obtain standing radiographs whenever possible to assess intraarticular wear or ligamentous laxity contributing to the overall deformity (see Fig. 23-8).

Fourth, if there is a suspected leg-length discrepancy, or 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. 23-2).

Digital versus Plain Radiographs

Digital radiographs have become the standard in many institutions. Standing anteroposterior long-leg radiographs using conventional analog printed film is being replaced by digital radiographs viewed on computer screens. Light boxes, rulers, goniometers, and grease pencils are being exchanged for computers screens that use graphics software to obtain measurements.

A wide variety of digital graphics software is available with most picture archiving and communication systems (PACS). Basic measurement tools typically include three-point angular measurements, Cobb angle measurements, digital rulers, and a calibration tool. Becoming familiar with these tools is critical to successful deformity planning.

The reliability and reproducibility of measurements obtained from digital versus conventional radiographs has been validated in multiple studies.20,21,37,55,60 In general, digital radiographic measurements are as accurate or improve accuracy and save time compared with conventional printed radiographs.

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. 23-16).

Figure 23-16 Compensation for tibial varus (A) or valgus (B) deformity with subtalar motion. LDTA, lateral distal tibial angle.

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 23-1).

Table 23-1

Normal Joint Motions Compensatory to Different Distal Tibial Deformities

Distal Tibial Deformity	Compensatory Motion	Common Compensatory Range
Varus	1° subtalar eversion	15°
	2° forefoot pronation
Valgus	1° subtalar inversion	30°
	2° forefoot supination
Procurvatum	1° ankle dorsiflexion	20°
	2° knee hyperextension
Recurvatum	1° ankle plantar flexion	50°
	2° knee flexion
Internal torsion	1° hip external rotation	Varied
	2° forefoot pronation
External torsion	1° hip internal rotation	Varied
	2° forefoot supination

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 as 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.

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. 23-17).

Figure 23-17 Osteotomy at center of rotation of angulation (CORA). Postoperative radiograph of patient shown in Figure 23-5. Osteotomy performed at CORA, followed by gradual correction with a Taylor Spatial Frame. The bone is accurately realigned with a lateral distal tibial angle (LDTA) of 90 degrees and no secondary translation.

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. 23-18). If there is a distal valgus deformity, the distal segment is translated laterally along with the angular correction.

Figure 23-18 Osteotomy proximal to center of rotation of angulation (CORA). Postoperative radiograph of patient shown in Figure 23-6. Osteotomy is performed proximal to the CORA, followed by gradual correction with a Taylor Spatial Frame. The bone is accurately realigned with a lateral distal tibial angle (LDTA) of 90 degrees, but there is a secondary medial translation.

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 Correction

Gradual correction of distal tibial deformities involves the technique of distraction osteogenesis, which is the formation of new bone after an osteotomy using tension lengthening techniques. Distraction osteogenesis – when used with multiplanar ring external fixation techniques – is a minimally invasive method, that is important when there is a compromised soft tissue envelope or compromised bone. Ring fixation techniques allow gradual simultaneous multiplanar correction, which is difficult to obtain with acute correction. Newer ring fixation devices, such as the Taylor Spatial Frame (Smith & Nephew, Memphis, Tenn.) correct complex deformity in a more efficient and reliable manner. Gradual deformity correction allows ongoing postoperative assessment of alignment, and adjustments are continued until accurate correction is achieved.

The decision to use distraction osteogenesis or acute correction may be a difficult decision, and multiple factors are considered. These include surgeon experience and ability with the various techniques, as well as careful patient evaluation.

In our experience, distraction osteogenesis is typically chosen over acute correction when the deformity is complex, often involving an oblique plane, a rotational component, and/or limb shortening. Also, gradual correction is chosen when a deformity is particularly severe and the ability to confidently correct it acutely is compromised. Other factors include prior infection or soft tissue envelope compromise, when an acute correction is likely to result in stretch of the neurovascular structures (particularly important in the varus to valgus correction), when significant shortening would occur, or when acute correction would create an unstable osteotomy.

The use of ring fixation, with or without gradual correction of deformity, may allow the patient to be more functional during the healing period because a ring fixator will typically allow partial to full weight bearing during the recovery time. Associated ankle arthritis is another consideration because the ankle joint can be distracted simultaneously with distal tibial deformity correction.

The decision to use distraction osteogenesis techniques and/or ring external fixation includes a careful evaluation of the patient and the patient’s ability to care for a ring fixator. Because patient involvement is required during the treatment, we favor an organized team approach in which all people involved are familiar with the ring fixation techniques, including the orthopaedic surgeon, office staff, physical therapist, orthotist, and hospital staff, both in the operating room and patient care units.

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.⁵⁶ The use of power saws should be avoided in the tibia.¹⁵ 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 preferred in the metaphyseal regions of the tibia. With gradual correction techniques, the osteotomy is usually completed after an external fixator has been applied.