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Locking mechanism failure between tibial baseplate and polyethylene insert in cruciate retaining total knee arthroplasty

Published:September 02, 2022DOI:https://doi.org/10.1016/j.jcot.2022.102013

      Abstract

      We describe a case of total knee arthroplasty performed with one type of cruciate retaining knee prosthesis (Vanguard CR, Zimmer Biomet), wherein failure of locking mechanism occurred between the tibial baseplate and the polyethylene insert 10 months after initial surgery. While there are very few case reports in literature describing such locking mechanism failure, all of these have been in constrained and posterior stabilized knees. This case, to the best of our knowledge is the first such case seen with a cruciate retaining knee design. A 73-year old lady, having undergone left total knee arthroplasty with the above-mentioned prosthesis, presented 10 months after the index surgery with a 3-week history of worsening knee pain and medially protruding hardware. She mentioned an episode of minor twisting of her knee after which she developed pain and swelling over the medial aspect, and was found to have the locking bar backed out on X-rays. She underwent revision surgery, wherein the locking bar was found to have buttonholed through the medial capsule. She underwent polyethylene liner exchange, with a new locking bar. There was no evidence of infection. Her symptoms fully settled following revision surgery.

      Keywords

      1. Introduction

      The locking mechanism between the tibial baseplate and polyethylene liner is crucial for a successful modern knee arthroplasty prosthesis.
      • Jayabalan P.
      • Furman B.D.
      • Cottrell J.M.
      • Wright T.M.
      Backside wear in modern total knee designs.
      It is known that micromotion between the insert and tibial baseplate can result in backside wear, which increases the risk of osteolysis.
      • Wasielewski R.C.
      The causes of insert backside wear in total knee arthroplasty.
      The locking mechanism is hence important to keep this relative motion between these components to a minimum, while also allowing easy assembly and disengagement of the modular components for surgical ease.
      • Wasielewski R.C.
      The causes of insert backside wear in total knee arthroplasty.
      A previous study looking at the correlation between type of locking mechanism and backside wear, describes the types of locking mechanisms as peripheral or central, and using a tongue-in-groove or a dovetail mechanism.
      • Łapaj Ł.
      • Mróz A.
      • Kokoszka P.
      • et al.
      Peripheral snap-fit locking mechanisms and smooth surface finish of tibial trays reduce backside wear in fixed-bearing total knee arthroplasty.
      The Vanguard (Zimmer Biomet) knee design employs a locking bar (tongue-in-groove type) inserted from medial to lateral after applying the polyethylene insert onto the baseplate. The rationale of the mechanism is to compress the polyethylene insert against the baseplate by using this relatively oversized bar, which is meant to force the insert against a 10-degree posterior boss. The proposed advantage of this mechanism is that it allows for maintained thickness of the polyethylene insert on the peripheries, as the locking mechanism is kept anteriorly within the intercondylar area.
      Failures of such locking mechanisms have been described in a limited number of case reports,
      • Rapuri V.R.
      • Clarke H.D.
      • Spangehl M.J.
      • Beauchamp C.P.
      Five cases of failure of the tibial polyethylene insert locking mechanism in one design of constrained knee arthroplasty.
      • Tohgi T.
      • et al.
      Two cases of failure of the tibial polyethylene insert locking mechanism in one design of total knee arthroplasty.
      • Ay G.R.
      • D S.R.
      • T C.
      • C K.
      • E K.K.
      Failure of polyethelene insert locking mechanism after a posterior stabilised total knee arthroplasty- A case report.
      all of these being in constrained or posterior stabilized knee designs. These are believed to be due to forces on the locking mechanism caused from the constrained design of the prostheses.

      2. Case report

      A 73-year old lady with a background of rheumatoid arthritis on disease modifying drugs, osteoporosis and hypertension presented to our institution with a painful left knee, and was found to have predominantly medial compartment osteoarthritis with a varus deformity (Fig. 1). Her left knee range of motion pre-operatively was between 5 and 100°. She underwent a total knee replacement using a Vanguard CR knee prosthesis, with a 12mm polyethylene insert (Fig. 2). Her initial post-operative course was uneventful, but she did describe feeling tightness at the posterior aspect of her knee, which was uncomfortable, but did not affect function. At her 6-week post-operative visit, she was found to have range of motion from 5° to 110°. It was noted that she had a reversed tibial slope on the post-operative X-rays.
      Fig. 1
      Fig. 1Pre-operative radiographs showing predominantly medial compartment osteoarthritis.
      Fig. 2
      Fig. 2Immediate post-operative radiographs. Reversal of tibial slope is noted.
      Approximately 10 months after the surgery, she developed pain in her operated knee, which she attributed to a minor episode of twisting. She also noted appearance of swelling on the medial aspect of the knee (Fig. 3). Given worsening pain and persisting swelling, she reported to us 3 weeks after the onset of pain, and backing out of the locking bar medially, was noted on X-rays (Fig. 4). She was noted to have no restriction of knee range of motion, which remained between 5 and 110°. Infection was ruled out clinically as well as with blood markers for infection, which were all normal. Her operation notes were reviewed, and there were no concerns noted at the time of surgery regarding locking of the polyethylene liner.
      Fig. 3
      Fig. 3Clinical picture showing medial sided swelling, with hardware palpable in the subcutaneous tissues.
      Fig. 4
      Fig. 4Radiographs 10 months following index surgery showing backing out of the locking bar.
      She underwent revision surgery on an urgent basis to avoid further backing out of the bar resulting in soft tissue compromise. The same medial parapatellar approach was used to expose the knee, and the locking bar was found to be buttonholed through the medial capsule. No wear of the polyethylene liner was noted on either of its surfaces, and it was found to be well seated on the tibial component with no evidence of subluxation. Given her description of posterior tightness and intraoperative judgement of gaps, she underwent polyethylene exchange to an insert one size lower (10mm). A new locking bar was inserted (Fig. 5). All her symptoms had subsided at final follow up, including that of feeling tightness in the posterior aspect of the knee. Her knee range of motion remained unchanged from the pre-operative range.
      Fig. 5
      Fig. 5Final X-rays following exchange of polyethylene liner to one size lower, and new locking bar.

      3. Discussion

      To our knowledge, this is the first such complication encountered with a cruciate retaining design of knee prosthesis. All case reports in literature thus far on failure of locking mechanisms have been for constrained or posterior stabilized knees. It is theorized that constraint induces greater micromotion between the tibial baseplate and the polyethylene insert, leading to an increased propensity for the locking mechanism to disengage. A previous case report has described a failure of the same locking mechanism, but this was notably in a posterior stabilized knee, and occurred early following surgery (6 weeks). The rate of such a complication has been reported to be 0.008% in the manufacturer's records.
      • Thienpont E.
      Failure of tibial polyethylene insert locking mechanism in posterior stabilized arthroplasty.
      Hepinstall et al.
      • Hepinstall M.S.
      • Rodriguez J.A.
      Polyethylene subluxation: a radiographic sign of locking mechanism failure after modular total knee arthroplasty.
      described a radiological method after a retrospective review, whereby they describe anterior subluxation of the liner relative to the baseplate, to identify failure of the locking mechanism. This was for posterior stabilized PFC Sigma knees (Depuy) and is a useful method, but is not applicable to the Vanguard knee design. Moreover, backing out of the locking bar in this system does not necessarily mean subluxation of the insert, as part of the bar still holds the insert in place.
      Rapuri et al.
      • Rapuri V.R.
      • Clarke H.D.
      • Spangehl M.J.
      • Beauchamp C.P.
      Five cases of failure of the tibial polyethylene insert locking mechanism in one design of constrained knee arthroplasty.
      describe five cases of locking mechanism failure with the Nex Gen LCCK (Legacy Constrained Condylar Knee) design. This system uses a screw to fix the insert into the tibial baseplate. They postulated that counterclockwise micromotion of the femur relative to the tibia during the screw home mechanism in terminal extension, results in loosening of this screw. The case report by Thienpont
      • Thienpont E.
      Failure of tibial polyethylene insert locking mechanism in posterior stabilized arthroplasty.
      describes failure of the Vanguard (Zimmer Biomet) locking bar in a PS knee. This occurred early (6 weeks post operatively), and the polyethylene liner was retained.
      In our case, there is no evidence to suggest surgical error in locking of the components, given that this failure occurred late in the post-operative course. Evidence from biomechanical models suggests that had this been the case, a loose locking bar would have backed out much sooner, given that the patient was weight-bearing normally in the post operative period. The reversed tibial slope following surgery may have possibly contributed to altered femoro-tibial contact forces, resulting in disengagement stresses on this locking mechanism, and this has been described as a risk factor in literature for such failure.
      • Thienpont E.
      Failure of tibial polyethylene insert locking mechanism in posterior stabilized arthroplasty.
      Also, the patient's complaint of posterior knee tightness suggests that an insert of a lower thickness may have been a better biomechanical fit. This was corroborated intra-operatively during revision surgery, where a 10mm liner was found to be stable with a good range of knee motion.
      The main learning point from this case report is that failure of locking mechanism can occur in CR knee designs, and is not limited to PS and constrained designs as previously described. The rarity of such an occurrence makes it difficult to understand the causes for such a failure, but a reversed tibial slope combined with a relatively high thickness of polyethylene insert in a CR knee possibly contributed to such a failure.

      Funding

      This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

      Declaration of competing interest

      The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

      Acknowledgements

      None.

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