Patient-Specific Instrumentation In Total Ankle Replacement

Over the past two decades, total ankle replacement (TAR) has challenged ankle arthrodesis (AA) as the “gold standard” for end-stage ankle arthritis.¹ The allure of TAR is multi-factorial, but suffice it to say, both surgeon and patient seem to favor TAR since this preserves some motion of the ankle joint complex and reliably relieves pain in the short- and medium-term to the same extent as ankle arthrodesis.^1-5 The potential to minimize progression of adjacent midtarsal and subtalar joint degenerative joint arthritis following TAR compared with ankle arthrodesis remains an unproven but appealing advantage.^6-11

What is clear is that anatomic alignment following both ankle arthrodesis and TAR is obligatory for long-term success.^1,12-14 Specific to TAR, malaligned metallic components demonstrate subsequently increased peak component pressures, potentially leading to component loosening, mechanical failure, need for revision and overall worse clinical outcomes.^15-24 The protracted learning curve associated with mastering TAR performed with standard extra- or intra-medullary reference guide (SRG) systems led to the introduction of preoperative computed tomography (CT) scan-based, engineer-provided plans with patient-specific pinning blocks or cutting guides (“patient-specific instrumentation” (PSI)).²⁵ There are four TAR systems released into the US market since 2012 that employ some version of preoperative CT scan-based, engineer-provided, planning-based PSI. These include, in alphabetical order: APEX 3D^™ TAR System with MAVEN (Paragon 28); PROPHECY^™ Preoperative Navigation Guides (Wright Medical Technologies, Inc./Stryker Orthopaedics)^{26, 27}; QUANTUM^® Total Ankle Prosthesis with OrthoPlanify^™ (In2Bones)²⁸; and VANTAGE Active Intelligence^® Ankle (Exactech).

Using Patient-Specific Instrumentation Systems: What You Should Know

Each TAR PSI system available for use in the US involves similar preoperative steps. The surgeon relies on the patient’s CT scan images obtained according to data collection protocol to develop a patient-specific, engineer-provided preoperative plan. In doing so, one determines metallic prosthetic component alignment and sizing using the particular patient’s unobstructed anatomic ankle joint landmarks or the adjacent tibiotalar surface anatomy. If one chooses, the surgeon may adjust the plan according to their own preferences and any additional patient particulars, such as retained hardware or cyst proximity to the intended TAR metallic components.

Once the surgeon approves the plan, the manufacturer creates three-dimensional (3D) models of the patient’s anatomy. From these models, they fabricate non-sterile, disposable pinning blocks or combined pinning/cutting guides to assist the surgeon in reproducing the surgical plan during the actual operative procedure. When deemed accurate intraoperatively, the surgeon avoids the task of intra- or extra-medullary guide set-up since the patient-specific pinning/cutting guides do not need further referencing.^25-28

In addition to improving metallic component alignment and thus risk of revision,²⁵ additional benefits claimed by a US TAR PSI market leader include less procedural complexity, less intra-operative radiation exposure for operating room participants, as well as reduced surgical time.²⁹ They state that these potential benefits are due to the avoidance of intra- or extra-medullary jigs and the steps necessary to align the SRG. As of May 2019, surgeons had performed 21,222 total ankle replacements involving the PROPHECY PSI for the INFINITY and INBONE II systems in the US.³⁰ To date, the majority of publications for this system included authors with potential bias, specifically, those involved with the development of, and/or who provide consultation services for the parent company that markets the TAR systems in question as well as the proprietary CT scan-based technology platform (ConforMIS).^31-44 To the authors’ knowledge, none of the other TAR PSI systems have any clinical data published to date.

In order to justify the time and increased financial costs associated with a technique that seeks to shift the balance of surgical planning away from the surgeon and to the manufacturer-employed engineer-provided, CT scan-based planning, total ankle replacement PSI systems must demonstrate significant and consistent advantages over SRG. The most crucial factor relates to alignment due to the negative effect malalignment has on long-term TAR survivorship. Unfortunately, this data has not been produced. To the contrary, tibial component alignment in the frontal and sagittal planes is consistently ± three degrees for TAR performed with SRG and the studied PSI TAR systems.^31-42

Escudero and colleagues noted a low incidence of perfectly aligned primary INFINITY TARs performed with PSI and failed to identify a significant difference in the rate of “perfect” alignment between the 51 PROPHECY PSI and 16 SRG-performed INFINITY TARs.⁴¹ What complicates frontal plane measurements is the difference between surgeon preference for mechanical versus anatomical tibial axis referencing. This is important because one cannot measure the mechanical axis on plain film radiographs. So, if one uses the mechanical axis preoperatively while using the anatomical axis postoperatively, then the surgeon would need to include the difference between these two axes when calculating the frontal plane accuracy. Unfortunately, none of the published data includes this information despite most authors noting use of the mechanical axis preoperatively and the differences between the mechanical and anatomical axes being ± two degrees.^30-44

Published data on the axial rotational alignment of the PROPHECY PSI INBONE II TAR system tibial component demonstrates poor accuracy of 50 percent within ± three degrees and 77 percent within ± five degrees.^45-48 Ultimately, accurately assessing axial rotation during TAR remains problematic due to a wide variation in the rotational alignment of the tibia, which one cannot accurately assess clinically or with plain film radiographs.^45-48 Further, the pre- and postoperative accuracy of talar component placement remains unknown, since the available studies involving these systems have not studied this specifically. Previous studies demonstrated that the PROPHECY PSI for the INFINITY or INBONE II primary TAR systems did not correctly predict the size or length (standard versus long) of the tibial component nor the size of the talar component implanted.^36-38,44 Additionally, similar to prior publications, the difference between the templated or alternative tibial and talar components and the one actually implanted were most often one size smaller.^36-38,44 This may be due to the variable medial and lateral gutter debridement that is dependent on direct intraoperative visualization and the ultimate polyethylene thickness required for the surgeon to tension the periarticular soft tissues.

Considering Factors Related To Cost Analysis

Most publications involving the PROPHECY PSI for the INFINITY or INBONE II TAR systems include secondary procedures necessary to structurally realign the foot and balance the periarticular ankle soft tissues, correct equinus contracture, or address adjacent degenerative joint disease. Although commonly performed, these secondary procedures do not factor into the manufacturer-employed engineer, CT scan-based planning in these systems. This remains a weakness of total ankle replacement performed with patient-specific instrumentation, especially when discussing cost analysis.

Cost analysis for TAR PSI versus SRG is complex and rife with potential for strategic manipulation but remains integral to understand, although studies are limited. One study that employed “micro-costing techniques” using published costs and institutionally acquired data yielded an operating room cost of $23.20 per minute in 2014/2015 United States dollars.⁴⁴ These authors identified a four-minute reduction in operating room time set up and an additional 38-minute reduction in surgical time using the PROPHECY PSI, amounting to an $863.00 savings per case in 2014/2015 United States dollars. However, the operating room cost analysis did not include the cost of the PROPHECY preoperative CT scan-based, engineer-provided plans and pinning guides.⁴⁴

Another cost analysis study compared traditional operating room cost accounting with “Time Driven Activity Based Cost” (TDABC) and determined the use of SRG resulted in an increase of $7,597.00 in 2016 United States dollars for the traditional method and a savings of $836.00 in 2016 United States dollars using the TDABC method.⁴³ This discrepancy highlights the potential for strategic manipulation of cost analysis data to favor one method over another, depending on the accounting data that is selected. Although likely facility-specific, there is a direct financial cost of approximately $1,500.00 in 2016 United States dollars passed on to the end-user for the company to create the CT scan-based, engineer-provided plan and pinning blocks.⁴³ Additional costs not included in this fee involve the cost of the preoperative CT scan (approximately $1,275.00 in 2014/2015 United States dollars),⁴⁴ as well as the sterilization costs for the patient’s 3D model and pinning blocks of approximately $100.00 per tray in 2011 United States dollars.⁴⁹

Finally, immeasurable costs exist that involve the surgeon’s time required to submit, review, and modify the preoperative plan, as well as the surgeon’s time required preoperatively to practice the “surface match” orientation of the pinning blocks on the 3D-printed non-sterile model to then replicate on the patient intraoperatively. This is an important consideration because the “surface match” between the patient-specific pinning blocks and underlying bone represents a critical step in the ultimate accuracy obtained when employing PSI for TAR, but remains unstudied in clinical practice. Only one other study, an unpublished master’s thesis in mechanical engineering, evaluated the PSI “surface match” accuracy.⁵⁰ The author identified a persistent mismatch between the PROPHECYPSI pinning block and the tibia adjacent to the ankle joint line through finite-element analysis. This is important since the ankle joint line and associated distorted surface anatomy is what one uses to determine ideal alignment during application of the PSI blocks. Additionally, the author determined that the amount of force required to overcome shearing between the pinning block and the underlying bone exceeded the deforming force of the pinning block material.⁵⁰

When taken collectively, it is not surprising to see that published reports,^35,36 as well as the US Food and Drug Administration (FDA) “Manufacturer and User Facility Device Experience (MAUDE)” describe multiple reports of surgeons abandoning the PROPHECY PSI and converting to the SRG for the INFINITY or INBONE II TAR systems in order to complete the surgery safely.⁵¹

Multiple questions remain related to the pinning blocks, such as the potential deformation that likely occurs during the manufacturing process, sterilization at the surgical facility, and application to the bone during the “surface match” process. Regardless, it is obvious that the surgeon should repeatedly practice the steps involved for the proper “surface match” process to most closely align with the preoperative CT scan-based, engineer-provided plans. Unfortunately, the actual amount of time the surgeon spends assessing the pinning block “surface match” preoperatively, as well as their frequency of actually revising the CT scan-based, engineer-provided plan, remains unknown. The time lag between obtaining the CT scan, preparing the engineer-based plan, and performing the surgery is important since the ankle joint anatomy used for the “surface match” process may potentially change between obtaining the CT scan and the date of surgery. Additionally, the ability of an engineer to accurately plan a complicated operation without clinical background information and unknown experience directly related to total ankle replacement is not yet established. Unfortunately, at this time, manufacturers have not released any data specific to their engineers’ experience and the accuracy of TAR placement performed with PSI.

Concluding Thoughts

Despite nearly 10 years and more than 21,000 PROPHECY PSI for the INFINITY and INBONE II TARs performed in the US³⁰ and three additional PSI-based TAR systems released in 2021, the theoretical benefits remain a matter for conjecture. Taken collectively, the available data cannot support the manufacturer’s claims of improved prosthetic implant accuracy, less procedural complexity, less intra-operative radiation exposure for operating room participants, and reduced surgical time with the use of TAR performed with PSI compared with SRG. The same holds true for primary total hip,⁵² knee,^53-56 and shoulder^57,58 replacement. It is noteworthy that the interest in PSI for total hip, knee, and shoulder replacement peaked in the early 2010s and has essentially since been abandoned. We believe this supports our concept that PSI technology does not replace the need for joint replacement surgeons to master the SRG techniques.

Ultimately, we caution surgeons against blindly trusting the engineer-provided preoperative plan, since the recommendations and pinning guide “surface match” errors may lead to malpositioning and sizing of the components necessitating intraoperative conversion to the TAR system’s SRG, which the surgeon may be unfamiliar with utilizing. Ideally, the manufacturers of TAR PSI systems will release their data for analysis by unbiased parties to help clarify the real from the perceived benefits attainable. The release of this information would also allow further evaluation and improvement in engineered systems to better assist surgeons in performing TARs, especially those requiring secondary procedures.

Prior to widespread adoption of any preoperative CT scan-based TAR PSI planning system, we advocate for non-biased surgeon publication of clear patient, surgeon, and facility benefits over SRG. Furthermore, meaningful cost analysis should include the entire TAR experience for the patient, surgeon, and facility, starting with the initial patient consultation and terminating at surgical completion rather than selectively excluding portions that may not favor CT scan-based TAR PSI planning systems. Finally, manufacturer marketing should use caution when advocating one TAR system’s superiority over another as the data for new technology remains unproven or risk succumbing to the same fate as PSI used for other orthopedic joint arthroplasties. 

Dr. Roukis is a Fellow and Past President of the American College of Foot and Ankle Surgeons and is a Professor in the Department of Orthopaedic Surgery and Rehabilitation, Foot and Ankle Service Line at the University of Florida College of Medicine-Jacksonville, in Jacksonville, FL.

Dr. Hyer is a Fellow of the American College of Foot and Ankle Surgeons. He practices with and is the Fellowship Co-Director at Orthopedic Foot and Ankle Center in Worthington, OH.

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