INFINITY/Prophecy Total Ankle Arthroplasty

50 INFINITY/Prophecy Total Ankle Arthroplasty

Steven L. Haddad

Abstract

Total ankle arthroplasty is an accepted and validated technique to treat end-stage ankle arthritis. Experience through the 1990s and early 2000s with longer-term follow-up of second-generation prostheses allowed surgeons to recognize failure through osteolysis and over-resection of the native bone leading to progressive implant subsidence. Lower-profile prostheses (especially the talus component) were then advocated to maximize residual talar bone at the time of implant revision. The INFINITY implant performs minimal resection of talar bone while allowing excellent implant stability (minimizing shear) through component interface with the native talus, and increased radius of curvature for coronal plane stability. Accuracy of implantation is enhanced through the Prophecy system, and it allows the surgeon to carefully plan the surgery prior to entering the operating room (decreasing operative time), minimizing the need for fluoroscopic assistance. Choosing implant size prior to surgery prevents over-resection of bone (and subsequent malleolar fracture) while allowing appropriate sagittal plane coverage to minimize the risk of subsidence. This chapter outlines the methodical approach to advance total ankle implantation through the INFINITY Prophecy system. Tips and pearls will assist the surgeon in maximizing operative efficiency, and hazards and bailouts are linked to prevent the surgeon from defaulting to ankle arthrodesis should intraoperative difficulties arise. Postoperative protocols are described to educate the surgeon on methodology to allow early range of motion without compromising the anterior surgical incision, ultimately creating fluid ankle movement without secondary scar contracture.

Keywords: ankle arthritis, total ankle arthroplasty, INFINITY implant, Prophecy system

50.1 Indications

• End-stage ankle arthritis.

• Ankle fusion on opposite extremity.

• Hindfoot fusion/significant adjacent joint arthritis.

• Adequate bone stock (nonneuropathic/Charcot’s bone destruction).

• Prior ankle fusion with symptomatic adjacent joint arthritis.

50.2 Pathology

• Ankle arthritis from one of three sources: primary osteoarthritis, posttraumatic arthritis, and systemic arthritis (e.g., rheumatoid, psoriatic).

• There was a time in the history of replacing ankles when one would avoid significant preoperative deformity, ligament instability, or bone loss. It is now clear that all of these pathologic states can be managed with ankle replacement through a structured plan (either single-stage or two-stage correction).

50.2.1 Clinical Evaluation

• The patient is observed standing for malalignment of the ankle and hindfoot, and the full limb is assessed for tibia vara or genu valgum (Fig. 50.1).

• Any malalignment must be assessed and measured prior to surgical procedure to assist with planning. Structural alignment of the foot is also assessed weight-bearing for either cavovarus foot or pes planovalgus.

• Weight-bearing stretch (dorsiflexion and plantar flexion) is measured with the examiner on the floor to increase accuracy.

• When viewed from behind, heel alignment is measured, and the patient is asked to rise onto tiptoes to assess both Achilles tendon strength and posterior tibial tendon strength (and ankle stability).

• Seated, the physician performs a neurologic examination to assess for neuropathy and assesses anterior and posterior pulses (and capillary refill) to evaluate vascular status. If either of these assessments creates concern, advanced testing is mandatory.

• The examiner documents prior incision placement, especially in posttraumatic cases, given that this may dictate safety and location of the anterior ankle incision approach. Quality of the skin is assessed for healing potential, and swelling documented to consider compression wraps to minimize this pathology prior to surgery (and lower tension on surgical incisions).

• Documentation of forefoot deformity assists with foot alignment assessment.

• Finally, gait is assessed to look for external rotation, antalgic pattern, and the potential for ankle replacement to have value in improving the patient’s gait.

50.2.2 Radiographic Evaluation

Weight-bearing radiographs of the foot and ankle are mandatory to understand not only the arthritis (and deformity) at the ankle, but also the alignment of the foot (Fig. 50.2). I also obtain a weight-bearing hindfoot alignment view to assist with deformity correction.

• I prefer a computed tomography (CT) scan over a magnetic resonance imaging (MRI) for assessment of ankle (and adjacent joint) arthritis because this gives a better interpretation not only of the arthritic condition, but also of the quality of bone within the ankle joint (location of cysts, and need for bone grafting following resection for prosthesis implantation).

• I prefer a diagnostic ultrasound for soft-tissue assessment (over an MRI) because this test is dynamic and can discern soft-tissue contracture that may require release or repair at the time of the replacement.

• In addition, for Prophecy cutting block guide design, a CT scan formatted per protocol including both the ipsilateral knee and ankle is necessary to determine the mechanical and anatomic axis of the extremity.

Fig. 50.1 Assessment of clinical alignment of the involved patient with left hemophilic ankle arthritis. (a,b) Standing alignment assessed and compared to opposite (right) side. (c) Hindfoot alignment assessed for varus/valgus heel. (d,e) Flexion/extension measured for comparative purposes.

50.2.3 Nonoperative Options

• Brace management (“Arizona” brace, or more commonly solid nonarticulated ankle foot orthosis).

• Injection of cortisone (preferably under fluoroscopy) for both diagnostic and potentially therapeutic information. These injections should be given no sooner than every 4 months.

• Physical therapy has a limited role, and in fact can aggravate symptoms in an arthritic ankle.

50.2.4 Contraindications

• Prior deep infection/osteomyelitis, though the temporal nature of prior infection should be assessed, and under certain circumstances bone biopsies done in advance of ankle replacement might determine lack of residual infectious process.

• Charcot’s or neuropathic bone destruction creates risk for component subsidence and failure (diabetes is not a contraindication, even with mild peripheral neuropathy).

• Severe bone loss (with the INFINITY implant). The INFINITY is a low-profile prosthesis and needs sufficient bone stock for structural support.

50.3 Goals of Surgical Procedure

• Pain relief, not only with ambulatory pain, but also with deep ache that wakes patient up at night.

• Improve motion at the ankle joint, which is now an achievable goal with ankle replacement.

• Lower risk of progressive arthritis developing at adjacent joints surrounding the ankle joint.

• Increase cadence and stride length from preoperative arthritic condition.

• Resume nonimpact activities such as hiking, skiing, and golf in a pain-free (or pain-improved) participant.

50.4 Advantages of Surgical Procedure

• The INFINITY implant is a low-profile prosthesis with the goal of preservation of sufficient bone stock to facilitate revision ankle replacement surgery in the future (and avoid defaulting to often unsuccessful ankle fusion using intercalary bone blocks).

• The INFINITY implant is fixed bearing, allowing multiplanar stability from the implant itself, helpful in grossly unstable patients.

• The INFINITY implant has coronal plane stability through the sulcus geometry of the polyethylene–talus component interface. This allows more aggressive gutter debridement, which improves motion and lessens tricompartmental ankle pain.

• The Prophecy system allows critical preoperative planning, determining the placement of the prosthesis prior to surgery, helping avoid surgeon misjudgment in both extramedullary and intramedullary alignment systems. Also, the surgeon can study the ability of the prosthesis to correct deformity without making the incision, and locate bone cysts that might cause late-term subsidence of the prosthesis. Finally, decisions on the need to remove preexisting hardware can be made in advance of the surgery.

Fig. 50.2 Assessment of radiographic alignment in the same patient. (a,b) Anteroposterior and mortise views to assess ankle alignment. (c) Standing axial calcaneus view to assess heel position with respect to the tibia and talus. (d) Standing lateral ankle/foot view to assess cavus or planus foot and apex of deformity to allow complete plantigrade correction. (e,f) Flexion/extension weight-bearing to accurately measure preoperative motion.

50.5 Key Principles

• Central incision placement.

• Limited exposure due to bone sparing implant.

• Prophecy system.

• Reduced reliance on fluoroscopy.

• Reduced operative time spent on component positioning.

• Simplified tibial tray preparation, single cut, and single broach system for fixation.

• Simplified talus preparation, one guide for chamfer cuts incorporates both anterior reaming and posterior cut under direct visualization of posterior structures.

• Mismatch tibia/talar implant size capability allows gutter resections to enhance motion.

50.6 Preoperative Preparation and Patient Positioning

• Standard weight-bearing radiographs of the ankle. This author also always obtains weight-bearing ankle flexion/extension views, three weight-bearing radiographs of the foot (to study alignment and need for supplementary osteotomies/arthrodesis), and a weight-bearing hindfoot alignment view (axial calcaneus).

Fig. 50.3 (a,b) CT (computed tomography) obtained with Prophecy protocol includes full ankle scan and knee scan.

Fig. 50.4 Example of preoperative Prophecy report, manufactured for the involved patient. This report illustrates the coronal sizing of the implant (a) and the sagittal sizing (b), allowing the surgeon to make preoperative adjustments in size and position of the prosthesis.

• Preoperative testing includes a Prophecy protocol CT scan (Fig. 50.3), which incorporates the knee and ankle joints to determine simulated mechanical and anatomic axes of the extremity, and is used to develop and manufacture Prophecy cutting block guides for the tibia and talus. The surgeon is responsible for reviewing the plan (Fig. 50.4) and approving it, and should study it carefully because it gives all information about proposed implant sizing, alignment, bone resection, and obstacles (i.e., hardware) that must be removed. This author also provides the company with weight-bearing radiographs to increase the knowledge base on alignment.

• If vascular insufficiency is suspected, preoperative venous and arterial Doppler exams are appropriate. If concern is present, an arteriogram is appropriate (especially in post-traumatic arthritis).

• Patient positioning is done with the ipsilateral hip bumped to bring the ankle to neutral (not internally or externally rotated), the heel at the end of the operating table, and the ipsilateral side as close to the border of the operating table as possible.

• Fluoroscopy is positioned on the ipsilateral side and the scrub tech/nurse is on the opposite side of the involved extremity.

50.7 Operative Technique

• Standard anterior surgical approach, normally 8 to 10 cm in length. The superficial peroneal nerve is defined and retracted. Though most state that the surgeon should not expose either the anterior tibial tendon or the extensor hallucis longus tendon, it is difficult not to do this. Thus, it is more important to preserve the overlying retinaculum for later repair.

• Retract deep peroneal nerve and anterior tibial artery on the lateral side. Elevate directly off bone to minimize injury to these structures.

• The surgeon must remove all periosteum, scar, fibrous tissue overlying the distal tibia bone to allow appropriate fitting of the cutting block guide (Fig. 50.5).

• Place the cutting block guide in the position determined by the report, and place one pin in the tibial portion of the guide. Place the longitudinal axis wire into the guide to check guide position under fluoroscopy (Fig. 50.6). Check central position of the guide and varus/valgus alignment. If confirmed, place the other three pins in the guide, and check one last time.

• Remove the cutting block guide, and place the Coronal Sizing Guide/Prophecy Conversion Instrument over these wires (Fig. 50.7). Use fluoroscopy to center the bull’s-eye on the guide, and again use the report as a reference to make sure the implant positioning is correct. Check to be sure the holes within the guide (which represent the corners of the tibial tray) do not violate the malleoli, confirming appropriate size. If you are “coupling” the tibia/talus cuts, bring the ankle to neutral and place two talus pins through the guide. Then, drill the two corner holes in the guide.

• Remove the Coronal Sizing Guide and place the actual cutting block over the wires (Fig. 50.8). Place gutter pins in the guide and cut all pins flush. Then, make saw cuts (tibia/medial/lateral, or with a coupled cut, tibia/medial/lateral/talus). Remove cutting block guides and all pins except the most proximal two tibial pins.

• For uncoupled cuts, denude the talus of any residual cartilage and/or fibrous tissue/scar tissue. Place the talar alignment guide (cutting block guide) over the talar neck, and pin in place with the oblique pin, followed by a single parallel pin. Check under sagittal fluoroscopy that the resection margin off the dome of the talus is correct. Then place the second parallel pin, remove the oblique pin, and place the cutting block back through the two remaining pins. Replace gutter pins and cut the talus. Remove the cutting block guide and remove as much talus bone as is accessible.