Comparison of tibial bone coverage of 6 knee prostheses: a magnetic resonance imaging study with controlled rotation

Transcription

1 Journal of Orthopaedic Surgery 2012;20(2):143-7 Comparison of tibial bone coverage of 6 knee prostheses: a magnetic resonance imaging study with controlled rotation Gregory C Wernecke, 1 Ian A Harris, 2 Michael TW Houang, 3 Bradley G Seeto, 1 Darren B Chen, 1 Samuel J MacDessi 1 1 Sydney Knee Specialists, Edgecliff NSW, Australia 2 South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, Australia 3 Castlereagh Imaging, Edgecliff NSW, Australia ABSTRACT Purpose. To compare the extent of tibial bone covered by the tibial tray in 6 most commonly used total knee arthroplasty designs in order to strike a balance between mediolateral cortical fit and optimal tibial component rotation. Methods. In 74 men and 27 women aged 17 to 60 (mean, 32) years with suspected soft-tissue injuries, their magnetic resonance images of the knee in full extension were superimposed with scans of the tibial trays of the 6 designs (one asymmetric and 5 symmetric). The tibial coverage by the tray and any posterolateral/posteromedial overhang/underhang were measured. Results. All 6 tray designs achieved tibial bone coverage of over 80%. Only 28% of all trays achieved optimal posterolateral fit, whereas 49% had posterolateral overhang enough to cause popliteal tendon impingement. Although the asymmetric tray provided highest tibial coverage (88%), its rates of relative and absolute posterolateral and posteromedial overhang were also highest (64%). Conclusion. The asymmetric tray provided improved tibial coverage at the expense of posterolateral and posteromedial overhang of the tibial tray. Key words: arthroplasty, replacement, knee; prosthesis design; rotation INTRODUCTION Optimising bone coverage with the tibial component in total knee arthroplasty (TKA) may avoid complications such as postoperative bone bleeding, subsidence, loosening, and component overhang. Many prostheses have a symmetrical tibial tray, although morphologically the medial tibial plateau is larger than the lateral one. 1 4 Anatomic or asymmetrical tibial trays have a smaller lateral plateau, which increases tibial coverage and reduces tibial overhang at the lateral corner. 5,6 It is unclear what extent of tibial overhang will lead to clinical symptoms requiring revision surgery. 7 Conversely, tibial underhang results in subsidence and Address correspondence and reprint requests to: Dr Gregory C Wernecke, Suite 211, New South Head Road, Edgecliff, NSW, 2027, Australia. dr4734@hotmail.com

2 144 GC Wernecke et al. Journal of Orthopaedic Surgery loosening in TKA, especially in uncemented tibial components Axial rotation of the tibial tray affects the extent of bone coverage. The best mediolateral or anteroposterior cortical fit of the tibial component usually does not result in optimal rotational alignment with the femoral component, which may adversely alter patellofemoral kinematics. 13 Based on the tibial alignment described by Insall, 14 complete medial and lateral cortical coverage often results in posterolateral overhang when using symmetric tibial components. Lessening the posterolateral overhang by internal rotation has a net effect of externally rotating the insertion of the ligamentum patellae. Increasing the Q-angle is associated with anterior pain, patellofemoral wear, and instability. 15,16 We compared the extent of tibial bone covered by the tibial tray in 6 most commonly used TKA designs in order to strike a balance between mediolateral cortical fit and optimal tibial component rotation. using the Osirix Dicom Viewer (Osirix Foundation, Geneva). An axial slice 8 to 10 mm distal to the lateral tibial plateau articular surface was chosen; this represented the average resection height during TKA (Fig. 1). The Insall line was then digitally drawn (Fig. 2). It was defined as a line from the junction of the medial (a) Insall line MATERIALS AND METHODS Ethics committee approved this retrospective magnetic resonance imaging (MRI) study. The tibial tray of the 6 most commonly used TKA designs (NexGen, LCS, PFC, Scorpio, Triathlon, and Genesis II) in the Australian National Joint Registry 17 were analysed. The first 5 were symmetric and the last one was asymmetric. Proton-density, fat-suppressed MRI scans of the knee in full extension from 74 male and 27 female skeletally mature patients (mean age, 32 years; range, years) who had suspected soft-tissue injuries were reviewed. All measurements were made (b) Insall line Optimal posteromedial fit Posterolateral overhang of 3.2 mm Figure 1 Measurement from lateral compartment articular surface (coronal view) and corresponding sagittal and axial cuts Figure 2 Axial magnetic resonance images of the proximal tibia are superimposed with (a) the Triathlon size 2.5 tibial tray showing good anterior, medial, and lateral fit with the tray rotated according to the Insall line, and (b) the Genesis II size 5R orientating towards the patella tendon demonstrating posterolateral overhang and a near perfect posteromedial fit. The Insall line is junction of the medial and middle third of the patella tendon to insertion of posterior cruciate ligament.

3 Vol. 20 No. 2, August 2012 Comparison of tibial bone coverage of 6 knee prostheses 145 and middle 1/3 of the tibial tubercle to the insertion point of the posterior cruciate ligament (PCL) on the posterior surface of the tibia. 14 The patellar ligament (rather than the tibial tubercle) was used as the anterior landmark, as the tubercle was distal to the plane of the axial slice. The Insall line would serve as the landmark in which the tray rotated. This was a highly reliable method for optimising tibial component rotation. All standard sizes of the 6 tibial tray designs were scanned together with a sizing marker to prevent magnification error. These digital images were then superimposed on the axial MRI slice of the knees (using Adobe Photoshop CS4), and transparency was adjusted to reveal any overhang and enable appropriate mediolateral sizing. The largest appropriate size to achieve mediolateral fit was chosen and rotated so that the anterior landmark of the tray was directed at the origin of the Insall line (Fig. 2). The posterior tail of the tray was manoeuvred to obtain equal medial and lateral coverage of the tibial surface. This often meant that the posterior notch of the tray was not precisely in line with the PCL insertion, but this centred the tray on the tibia. The femoral anteroposterior sagittal width of all knees was measured, and an appropriately sized femoral component was allocated to achieve femorotibial size matching, according to the sizing charts of the manufacturers. Some TKA systems have a variety of tibial components to match a specific femoral size. There was no mismatch in this series. For the NexGen prosthesis, the smaller of the 2 sizes was selected when the larger size had posterior overhang, because for every second size it increases in the anteroposterior distance but not the mediolateral distance. The tibial bone coverage of each tray design was calculated. This was defined as the total crosssectional area (CSA) of the appropriately sized tray minus any tray overhang, divided by the total CSA of the tibial surface. The maximal amount of posterior overhang and underhang was measured in well-fit trays (without anterior, medial, or lateral overhang) [Fig. 2]. Posterior overhang and underhang was measured separately on both lateral and medial sides and indicated as a positive and negative distance from the posterior edge of the tibia, respectively. A distance of 1 mm was defined as optimal sizing, 1 to 3 mm as relative overhang/underhang, and >3mm as absolute (unacceptable) overhang/underhang (Fig. 3). The popliteal tendon was identified along the posterolateral aspect of the tibia. The shortest distance between the anterior edge of the tendon and the posterior tibial cortex was measured. This indicated the extent of the overhung trays theoretically impinged on the popliteus in the extended knee (Fig. 4). In all patients, the popliteal tendon was noted to be posterior to the posterolateral corner of the tibia at the level of resection. Proportions were compared using the Chi squared test. Continuous variables between prostheses were compared using the paired t tests, as the same tibia was used for each pair. Tibial coverage by gender was compared using the un-paired t test. A p value of <0.05 was considered statistically significant. RESULTS All prostheses achieved tibial coverage of 80%. The asymmetric Genesis II tibial tray achieved significantly more tibial coverage than all other symmetric trays did (88% vs. 80%, p<0.001 for each comparison, paired t test, Table). The number of tray sizes available in each standard set did not correlate with total coverage (p=0.3). On average, only 28% of all tibial trays of the 6 designs demonstrated optimal posterolateral fit (Table). The rate of optimal posterolateral fit Absolute underhang (>3 mm) Relative underhang (1 3 mm) Optimal fit (±1 mm) Relative overhang (1 3 mm) Absolute overhang (>3 mm) Figure 3 Grading system for posterior overhang and underhang of the tibial tray. Distance between posterior tibial border and popliteal tendon (1.15 mm) Popliteal tendon Figure 4 An axial magnetic resonance image showing the popliteal tendon and the distance to posterior tibial edge.

4 146 GC Wernecke et al. Journal of Orthopaedic Surgery Design Table Comparison of tibial coverage and percentages of overhang and underhang of the tibial tray of the 6 designs* Tibial coverage (%) No. of sizes available Posterolateral/posteromedial fit of the tibial trays (% of patients) Absolute underhang (<-3 mm) Relative underhang (-3 to -1 mm) Optimal sizing (-1 to 1 mm) Relative overhang (1 3 mm) Absolute overhang (>3 mm) Symmetric NexGen (Zimmer) /90 33/8 34/2 22/1 5/0 LCS (Johnson & Johnson) /71 13/21 32/7 37/2 17/0 PFC Sigma (DePuy) /87 18/12 29/1 33/1 10/0 Scorpio (Stryker) /76 19/20 26/3 32/2 20/0 Triathlon (Stryker) /68 17/22 25/9 35/2 21/0 Asymmetric Genesis II (Smith and Nephew) /30 27/32 29/24 38/14 26/1 * Overhang is mainly a problem laterally and underhang is mainly a problem medially was highest for the NexGen (34%), which was not significantly higher than any other tray designs ranging from 25% to 32% (p>0.2 for all). The rate of relative posterolateral overhang was lowest for the NexGen (22%), compared to the Genesis II (38%, p=0.01), LCS (37%, p=0.02), Triathlon (35%, p=0.04), PFC (33%, p=0.07), and Scorpio (32%, p=0.1). The rate of absolute posterolateral overhang was lowest for the NexGen (5%), compared to the Genesis II (26%, p<0.001), Triathlon (21%, p<0.001), Scorpio (20%, p=0.001), LCS (17%, p=0.006), and PFC (10%, p=0.2). In total, 16% and 33% of all trays had absolute and relative posterolateral overhang (>1 mm), respectively. The mean distance of the popliteal tendon from the posterior tibial surface was 1±0.1 (range, 0 4) mm. On average, only 8% of all tibial trays of the 6 designs demonstrated optimal posteromedial fit (Table). The rate of optimal fit was highest for the Genesis II (24%), which was significantly higher than any other designs ranging from 1% to 9% (p<0.001 for all). The rate of relative or absolute posteromedial overhang was highest for the Genesis II (15%), which was significantly higher than for any other designs ranging from 1% to 2% (p<0.002 for all). 70% of all trays had absolute posteromedial underhang; the rate was highest for the NexGen (90%), compared to the PFC (86%, p=0.5), Scorpio (75%, p=0.01), LCS (70%, p<0.001), Triathlon (67%, p<0.001), and Genesis II (30%, p<0.001). DISCUSSION Asymmetric designs have been shown to have better proximal tibial fit. In a computed tomographic study of 4 symmetric and one asymmetric components in 72 knees, 5 asymmetric components had better fit. However, axial rotation was not controlled and the tibial trays were sized from the medial tibial plateau. In another study, symmetric designs have been shown to have better tibial coverage. 18 Better cortical fit on the tibial cut surface can be achieved with increasing number of tray sizes available. 18 However, the component rotation was also not controlled. In a recovery study of 42 tibial resection specimens with rotation being controlled, 6 the asymmetrically designed trays could be placed without overhang. It was concluded that the number of sizes available was more important than the shape of the tibial tray. 6 In a 5-year study of unicompartmental knee replacements, 7 pain and Oxford knee scores were significantly less favourable in patients with radiographically confirmed medial overhang of >3 mm. Our study had several limitations. Firstly, it was 2-dimensional and measurements could only be made in one plane. With the leg in full extension, this negated the slight increase in area that may be ensue in vivo with a built-in posterior slope cut. Secondly, the anterior landmark used for the Insall line was modified. We used the junction of the medial 1/3 and lateral 2/3 of the patella ligament at the level of the tibial resection. The true origin of the Insall line may be lateral to the point used in our study, resulting in underestimation of component external rotation and thus posterolateral overhang and posteromedial underhang. Thirdly, as patients were suspected to have soft-tissue injuries, joint effusions were present in some of the scans, and soft tissues were pushed away from the joint so as to increase the popliteusto-tibia distance. Therefore, our mean 1 mm distance between the popliteus and the back of the tibia may

5 Vol. 20 No. 2, August 2012 Comparison of tibial bone coverage of 6 knee prostheses 147 have been an underestimate, potentially increasing the risk of tendon impingement. Fourthly, we did not utilise all methods of tibial component rotation, only a method based on the location of the tibial tubercle. Fifthly, the gender distribution was not in keeping with arthroplasty registries. Our data (unpublished) indicate that the aspect ratios of the knee compartments are not gender dependent, despite females having smaller tibias than men. Finally, our patient cohort was relatively young and there were no degenerative changes or osteophyte formation. Although peripheral osteophytes surrounding the cut tibial surface are generally excised, it may potentially increase the average surface area of the tibia and thus decrease the tibial coverage. Nonetheless, the number of patients in our study was high. The use of MRI enabled direct measurement from articular cartilage surfaces (as is done intra-operatively) as opposed to the bone surface on computed tomography. Softtissue structures (such as the popliteal tendon) were easily visualised for assessment of tray overhang on soft-tissue impingement. Although the asymmetric tray provided highest tibial coverage, its rates of relative and absolute posterolateral and posteromedial overhang were also highest. In total, 48% of all tibial trays had posterolateral overhang of 1 mm that may result in popliteal tendon impingement. The advantages of improved tibial coverage by the asymmetric tray may be negated by the increased rate of posterior overhang. It is unclear what the minimum tibial coverage should be to avoid the risk of tibial component subsidence and failure. A higher failure rate has not been observed in symmetric trays that attain lower tibial coverage. Future design modifications to the tibial tray should address these issues. REFERENCES 1. Mensch JS, Amstutz HC. Knee morphology as a guide to knee replacement. Clin Orthop Relat Res 1975;112: Smith JR, Hofmann AA. Morphology of the proximal tibia in the arthritic knee. AAOS 59th Annual Meeting Washington DC, Feb Westrich GH, Haas SB, Insall JN, Frachie A. Resection specimen analysis of proximal tibial anatomy based on 100 total knee replacement specimens. J Arthroplasty 1995;10: Hitt K, Shurman JR 2nd, Greene K, McCarthy J, Moskal J, Hoeman T, et al. Anthropometric measurements of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am 2003;85(Suppl 4):S Stulberg BN, Dombrowski RM, Froimson M, Easley K. Computed tomography analysis of proximal tibial coverage. Clin Orthop Relat Res 1995;311: Westrich GH, Laskin RS, Haas SB, Sculco TP. Resection specimen analysis of tibial coverage in total knee arthroplasty. Clin Orthop Relat Res 1994;309: Chau R, Gulati A, Pandit H, Beard DJ, Price AJ, Dodd CA, et al. Tibial component overhang following unicompartmental knee replacement does it matter? Knee 2009;16: Goldstein SA, Wilson DL, Sonstegard DA, Matthews LS. The mechanical properties of human tibial trabecular bone as a function of metaphyseal location. J Biomech 1983;16: Nilsson KG, Karrholm J, Ekelund L, Magnusson P. Evaluation of micromotion in cemented vs uncemented knee arthroplasty in osteoarthrosis and rheumatoid arthritis. Randomized study using roentgen stereophotogrammetric analysis. J Arthroplasty 1991;6: Onsten I, Nordqvist A, Carlsson AS, Besjakov J, Shott S. Hydroxyapatite augmentation of the porous coating improves fixation of tibial components. A randomised RSA study in 116 patients. J Bone Joint Surg Br 1998;80: Nilsson KG, Karrholm J, Carlsson L, Dalen T. Hydroxyapatite coating versus cemented fixation of the tibial component in total knee arthroplasty: prospective randomized comparison of hydroxyapatite-coated and cemented tibial components with 5-year follow-up using radiostereometry. J Arthroplasty 1999;14: Carlsson A, Bjorkman A, Besjakov J, Onsten I. Cemented tibial component fixation performs better than cementless fixation: a randomized radiostereometric study comparing porous-coated, hydroxyapatite-coated and cemented tibial components over 5 years. Acta Orthop 2005;76: Figgie HE 3rd, Goldberg VM, Figgie MP, Inglis AE, Kelly M, Sobel M. The effect of alignment of the implant on fractures of the patella after condylar total knee arthroplasty. J Bone Joint Surg Am 1989;71: Insall JN. Surgical techniques and instrumentation in total knee arthroplasty. In: Scott WN, editor. Insall & Scott s surgery of the knee. 4th ed. Philadelphia: Churchill Livingstone, Elsevier; 2006: Malo M, Vince KG. The unstable patella after total knee arthroplasty: etiology, prevention, and management. J Am Acad Orthop Surg 2003;11: Parker DA, Dunbar MJ, Rorabeck CH. Extensor mechanism failure associated with total knee arthroplasty: prevention and management. J Am Acad Orthop Surg 2003;11: Australian Orthopaedic Association. National Joint Replacement Registry Annual Report Available at dmac.adelaide.edu.au/aoanjrr/documents/aoanjrrreport_2009.pdf 18. Incavo SJ, Ronchetti PJ, Howe JG, Tranowski JP. Tibial plateau coverage in total knee arthroplasty. Clin Orthop Relat Res 1994;299:81 5.