Recurrent Varus Following Total Ankle Arthroplasty

CHAPTER PREVIEW

CHAPTER SYNOPSIS:

This chapter will address the complicated problem of revising an ankle arthroplasty that has become a varus deformity following implantation. The implications of a sustained varus deformity following total ankle arthroplasty are grave for the implant, as eccentric loads about the prosthesis may cause further failure through subsidence and polyethylene wear. Thus, recognition of the problem and the knowledge base to successfully correct it are critical to survival of the implant.

IMPORTANT POINTS:

•

Determine if the varus deformity is coming from ligament instability, bone defects/subsidence, or mechanical alignment of the tibia/foot.
- ○
  
  Weight-bearing radiographs of the length of the tibia, the mechanical axis of the hip–knee–ankle, the ankle, and the foot are required, in addition to a Cobey view of the calcaneus.
•

Determine the potential to salvage the implant, versus removal and arthrodesis.
- ○
  
  A computed tomography scan is used to estimate the amount of residual bone present following implant removal/revision.
- ○
  
  Stress radiographs and reverse-stress radiographs are used to evaluate the ability to correct the implant to neutral.
•

Determine the best method to salvage the implant (if applicable).
- ○
  
  Ligament instability
  - ▪
    
    Revision to a custom implant will increase height (making up for bone loss) and improve ligamentotaxis.
  - ▪
    
    Reconstruct the deficient ligaments via a tendon transfer procedure.
  - ▪
    
    Alter the alignment of the foot and/or tibia to improve the mechanical axis and balance the load on the implant.
- ○
  
  Subsidence
  - ▪
    
    Revision to a custom implant will make up for bone loss and provide better coverage and fixation.
  - ▪
    
    Add reinforcement to stabilize the implant (bone graft versus polymethylmethacrylate).

VIDEO AVAILABLE:

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

As noted in the Chapter Preview, the implications of a sustained varus deformity following total ankle arthroplasty are grave for the implant, as eccentric loads about the prosthesis may cause further failure through subsidence and polyethylene wear. Thus, recognition of the problem and the knowledge base to successfully correct it are critical to survival of the implant. Fukuda, Haddad, and colleagues (Impact of Talar Component Rotation on Contact Pressure after Total Ankle Arthroplasty: A Cadaveric Study. Fukuda T, Haddad SL, Ren Y, Zhang L. Unpublished manuscript, 2009.) found that malrotation of the talar component prosthesis with respect to the tibial component leads to a statistically significant decreased contact area and subsequent increased peak pressure and rotational torque. This imbalance is similar to that seen with edge loading from angular deformity, and thus congruence between the tibial and talar components is paramount to a successful arthroplasty.

INDICATIONS

Varus deformity about a previously placed ankle prosthesis may arise from a variety of circumstances. Ligament imbalance is often the primary culprit, and in the case of varus, a persistent, deficient lateral ligament complex in combination with a persistent tight deltoid ligament allows deformity progression ( Fig. 14-1 ). This is rarely the sole component to recurrent varus deformity but must be considered during any attempt at revising the prosthesis. To assess ligament balance, the examiner may incorporate stress radiographs done manually. The examiner must also perform “reverse” stress radiographs to determine the capability of deformity correction. If the prosthesis does not correct to neutral alignment under these circumstances, then clearly there are other factors (i.e., bone impingement) creating the varus deformity, and isolated ligament reconstruction will fail.

A second and equally important factor is the mechanical alignment of the extremity, which may not have been addressed sufficiently in the primary arthroplasty. If the patient sustained a prior tibia fracture with valgus deformity (or if they have congenital genu valgum), alignment of the hip–knee–ankle axis invariably places a varus thrust about the ankle. Under these circumstances, patients with an existing implant fail through either subsidence of the prosthesis into varus by weakened medial bone structural support, or through attenuation of the lateral ligament complex with repetitive impact. Revision in this case thus involves either a custom prosthesis to restore height in the deficient bone or the ligament reconstruction mentioned earlier. In either case, it is critical to address the tibial deformity through osteotomy to prevent recurrence following revision. Discussion of these methods is beyond the scope of this text. Techniques range from opening or closing wedge osteotomies and, more recently, distraction osteogenesis to provide more fluid and adjustable correction. Needless to say, the goal is to establish a neutral mechanical axis to prevent aberrant prosthesis overload into varus. As with recurrent valgus deformity (see Chapter 15 ), the procedures are staged, with mechanical alignment reconstruction performed at least 3 months in advance of revision ankle replacement.

Along the line of tibial deformity creating varus, residual cavus or cavovarus in the foot will place increased structural demands on the prosthesis, allowing it to fail by one of the methods mentioned ( Fig. 14-2 ). Under these circumstances, the examiner must perceive the foot deformity (assisted by evaluation of the opposite foot weight-bearing), and correct said deformity through a closing wedge/lateral slide calcaneal osteotomy, dorsiflexion osteotomy of the first metatarsal, and perhaps a midfoot osteotomy through the navicular-cuneiform joints and cuboid to balance structural deformity. It is critical that the surgeon recognize this problem, as it is often the most unrecognized issue that leads to varus in an already implanted prosthesis. Again, correction of the foot deformities is beyond the scope of this publication, although this is not to underemphasize their importance in obtaining a balanced ankle prosthesis. The surgeon must be certain that the foot is plantigrade, and this must be done simultaneously with component revision, as there is no better time to appreciate the foot forces creating imbalance and varus deformity.

Following failure of the prosthesis into the supportive bone, the surgeon must consider osteolysis. Osteolysis occurs following shed polyethylene particles, creating a macrophage reaction and autodestruction of bone through osteoclastic reaction. This weakened, cystic bone allows the prosthesis to subside through the resection margin, creating deformity and failure. A preoperative computed tomography (CT) scan is important in making this assessment. All osteolytic cysts must be targeted for bone grafting following recognition.

Interesting compounding factors in revision for deformity revolve around the character of the bone (tibia and talus) following implant removal. This in and of itself is not a contraindication for surgical revision but must be respected and planned for during the reconstruction. This problem is more often noted on the talar side of the prosthesis, as tilt and subsidence erode the softer talar bone, leading to a void that must be considered when addressing fixation of the revised talar component. Stemmed implants are one option, of course, but this author has used polymethylmethacrylate in combination with bone grafting to allow increased accuracy with talar component placement. The longevity and reviseability of either construct, however, remain in question.

SURGICAL TECHNIQUES

Lateral Ligament Reconstruction

This technique is illustrated for deltoid ligament reconstruction in Chapter 15 .

Surgical ligament reconstruction is challenging due to the space occupied by the prosthesis. Thus, standard techniques need to be modified to accommodate the bone architecture provided by resection necessary for prosthesis implantation.

Patients with ligament incompetence are assessed for laxity via stress radiographs mentioned. Appropriate planning assesses the patient for hindfoot and forefoot deformities that may require simultaneous correction. Gross ligament laxity must be accounted for, as balance achieved through tightening one side of the ankle may create the opposite deformity from that corrected due to a lack of contralateral restraint. Finally, a diagnostic ultrasound may be used to assess the quality of the posterior tibial tendon and peroneal tendons, as magnetic resonance imaging will be compromised by the prosthesis.

The patient is placed supine on the operating table, with the leg aligned neutral (neither internally nor externally rotated).

The surgical approach requires two incisions if scar tissue debridement is necessary. The initial approach reproduces the standard anterior incision performed for the index ankle arthroplasty, maximizing the skin bridge to minimize wound complication. Exposure is carried proximal to the ankle joint, and any impinging soft tissue or bone is removed.

If no such debridement is required (i.e., the prosthesis corrects easily through varus stress radiographs), then the anterior approach is not required. Instead, a lateral approach is performed paralleling the fibula, traveling toward the fifth metatarsal base at the tip of the fibula.

Unfortunately, a modified Brostrom is not sufficient to stabilize the lateral ligaments in light of an ankle replacement. Thus, a similar cadaveric tendon transfer done for recurrect ankle replacement valgus is performed to stabilize the deficient lateral ligaments.

A cadaveric anterior tibial tendon or semitendinosis tendon is tubularized with nonabsorbable suture, and a Krakow suture with No. 2 Ethibond is weaved at one end of the tendon. An endobutton is secured to this suture with a 1-cm gap between the end of the tendon and the endobutton.

A drill hole is made through the talar neck at the insertion of the anterior talofibular ligament, exiting anterior to the medial malleolus. The far cortex of the hole is smaller than the length of the endobutton. The endobutton/tendon complex is then placed through this hole, and the endobutton is “flipped.”

If sufficient fibula is present distal to the lateral portion of the tibial tray, 7.3-mm drill holes are placed at the origins of the anterior talofibular ligament and calcaneofibular ligament. These holes meet in the central fibula. The allograft tendon is placed through these holes, with the distal segment exiting through the inferior (calcaneofibular ligament) hole. If there is insufficient fibula at the tip, the plate for securing the syndesmotic fusion is carried to the tip of the fibula, and the cadaveric tendon is placed deep to the place. A screw is placed in the most distal hole of the plate to assist with stabilizing the transferred tendon.

In either case, the hindfoot is placed into eversion, and a 4.5-mm drill hole is placed in the calcaneus, from lateral to medial, at the anatomic insertion of the calcaneofibular ligament. The cadaveric tendon is placed under maximal tension, and a knife is used to bisect the allograft proximal to the previously drilled hole.

A 6.5-mm fragment screw with a large spiked ligament washer is placed through the cadaveric tendon at the point of the previously placed incision. This screw is placed proximal to the previously drilled hole. Again, this technique will maximize tension on the transferred tendon. The screw is then inserted into the calcaneus, with the spiked washer completely engaging the tendon to provide rigid fixation ( Fig. 14-3 ).