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Two stage fracture of a polyethylene post in a 9-year-old posterior-stabilized knee prosthesis: a case report
© D'Angelo et al; licensee BioMed Central Ltd. 2010
Received: 4 November 2009
Accepted: 23 February 2010
Published: 23 February 2010
Several cases of tibial post breakage are reported in the literature. To the best of our knowledge, only three cases of NexGen knee prosthesis (Zimmer, Warsaw, Indiana, USA) tibial post failure have been reported.
In November 1999, a 63-year-old Caucasian woman from Italy with a history of symptomatic left knee osteoarthritis underwent a total knee arthroplasty. In March 2008, while rising from a chair, she felt a sudden pain and instability in her left knee. She reported a fracture of the polyethylene post of the tibial insert. No malposition or malalignment of either the femoral or tibial components were identified. The polyethylene tibial insert was studied under light microscopy and scanning electron microscopy. The fracture was also noted to have occurred without any notable polyethylene wear.
Scanning electron microscopy revealed two different damage patterns that could be explained with a two-stage rupture of our patient's polyethylene post. This could have been caused by a non-optimal ligamentous balancing during first implant surgery. Her knee probably developed a varus instability that weakened the post, and then a posterior anterior stress finally broke the polyethylene.
The interaction between the polyethylene post of the tibial tray and the femoral cam is necessary for the proper functioning of posterior stabilized (PS) knee prosthesis . PS total knee arthroplasty (TKA) was developed to grant stability, to achieve a higher range of motion due to rollback, and to prevent posterior subluxation of the implant . The polyethylene spine contacts the cam at approximately 70° of flexion, thus preventing posterior subluxation. Mediolateral stability, however, is dependent only on a well balanced and aligned knee .
Polyethylene wear is a complication that could contribute to aseptic loosening and osteolysis after TKA . Acknowledged factors that can influence polyethylene wear include prosthesis design, manufacturing, and poor surgical technique [5, 6].
Several cases of tibial post breakage are reported in the literature [7–13]. To the best of our knowledge, three cases of NexGen PS knee prosthesis (Zimmer, Warsaw, Indiana, USA) tibial post failure have been reported [14–16]. This case report focuses on light microscopy and scanning electron microscopy (SEM) evaluation of the broken polyethylene insert. This report also aims to explain a possible mechanism for the failure of tibial post.
In November 1999, a 63-year-old Caucasian woman from Italy (weight = 100 kg, height = 1.60 m, body mass index = 39) with a history of symptomatic left knee osteoarthritis underwent a TKA in another hospital. The implant used was a NexGen PS knee prosthesis (Zimmer, Warsaw, Indiana, USA) with a tibial component size of 4, a femoral component size of D, and a polyethylene insert 10 mm in thickness. No problem was reported during the follow-up examination, and the patient was able to perform normal life activities for the next nine years.
Our patient underwent diagnostic knee arthroscopy and was scheduled to have her polyethylene insert changed. The procedure granted a clear view of the polyethylene and the broken post in articulation. However, actual findings showed that the polyethylene had no relevant wear areas or alterations. Based on these findings, we performed an anterior approach with medial parapatellar arthrotomy. During surgery, samples of the periprosthetic tissue were taken in order to obtain a histological evaluation. These specimens were treated with haematoxylin-eosin and von Kossa staining, and were then studied under light microscopy using polarized light in order to detect the typical birefringence of the polyethylene debris.
The total knee components appeared to be well fixed intraoperatively. The polyethylene insert was substituted with a 12-mm CD LPS Flex articular surface (Zimmer, Warsaw, Indiana, USA). The stability in full extension, mid-flexion, and full flexion of the knee was tested intraoperatively, and appeared to be good. The patient had no postoperative complications and recovered well.
The design feature common to all PS knee prosthesis is the cam-and-post mechanism that is incorporated into the femoral and tibial components. The cam on the femoral component is designed to engage the post of the tibial polyethylene during knee flexion. This interaction provides a functional substitute for the posterior cruciate ligament (PCL), thus resulting in femoral rollback as flexion increases. In addition, the cam and the post work to limit posterior displacement of the tibia relative to the femur in extension .
In some cases in which the resulting laxity in flexion is greater than the so-called "jump distance", or the height of the post, acute dislocation may occur. Another potential cause of flexion instability in a knee with PS prosthesis is the failure of the polyethylene post. This can be caused by either polyethylene acute fracture or fatigue fracture, which is a consequence of repetitive anterior impingement between the metal femoral cam and the polyethylene post .
No malposition or malalignment of both the femoral and tibial components were identified in our patient. Confirming the findings of Colizza et al., , polarized light microscopy did not reveal any notable polyethylene wear.
The medial part of the fracture edge appears to be smooth (Figure 2 areas 2 and 3) and with a different orientation of the fracture lines. These characteristics suggest a chronic instability of the varus valgus knee prosthesis that slowly weakened the polyethylene post.
Especially in posterior stabilized designs, it is important to achieve a well-balanced and aligned knee in order to reduce stress on the polyethylene spine that could otherwise lead to fatigue fracture .
We believe that the major failure mechanism of the polyethylene post in our patient was the mild varus valgus instability related to a non-optimal ligamentous balancing during her first implant surgery. This aspect, together with our patient's weight, produced a progressive weakening of the polyethylene post, which finally broke due to hyperextension mechanism.
Based on the experience of Callaghan et al. , proper femoral component positioning and avoiding excessive posterior tibial slope during implant surgery is crucial to reduce the anterior impingement of the post. Our patient's tibial slope was only 2°, which indicates a good compromise between ROM and tibial post impingement.
For most patients, once the diagnosis has been established the revision of the polyethylene insert is mandatory when components are well-fixed and in good alignment. In our patient, an insert that was only 2 mm thicker was enough to restore the stability of her knee. However, if the components are loose or malpositioned, complete revision surgery is recommended.
When sudden pain and instability appear in a functioning knee PS TKA, a tibial post breakage must be considered.
Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
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