Patellar Component Research Articles

Unusual mechanical complications of unicompartmental low contact stress mobile bearing patellofemoral arthroplasty: A cause for concern?

The Low Contact Stress Patellofemoral Arthroplasty (LCS PFA) is a newer design belonging to the second generation of inlay type implants, addressing the problems encountered in the first generation models (Lubinus & Richard's). The cemented mobile bearing metal backed patellar component in this system is “modular”; allowing interchangeable usage with either the trochlear component in a PFA or the femoral component of a total knee arthroplasty, thus obviating the need for patellar revision during conversion of PFA to TKA. The younger active patient with a PFA may exert extreme joint reaction forces on their patellar implant, rendering early loosening of the patellar implant. The endurance of this implant with repeated flexion beyond 90° is also a concern. We describe a series of three unusual mechanical failures associated with this particular design of metal backed patellar component of the unicompartmental LCS patellofemoral arthroplasty.

Improving accuracy of total knee component cementation: description of a simple technique

BackgroundTotal knee arthroplasty represents a common orthopedic surgical procedure. Achieving proper alignment of its components with the predrilled patellar and tibial peg holes prior to polymerization of the bone cement can be challenging.TechniqueAfter establishing the femoral, patellar and tibial bone cuts, the cancellous bone around the tibial keel, as well as the peg holes for the patella and femoral components are marked with methylene blue using a cotton swab stick. If bone cement is then placed onto the cut and marked bone edges, the methylene blue leaches through the bone cement and clearly outlines the tibial keel and predrilled femoral and patellar peg holes. This allows excellent visualization of the bone preparations for each component, ensuring safe and prompt positioning of TKA components while minimizing intraoperative difficulties with component alignment while the cement hardens.ConclusionThe presented technical note helps to improve the accuracy and ease of insertion when the components of total knee arthroplasty are impacted to their final position.

Extensor Mechanism Ruptures

The extensor mechanism of the knee consists of the quadriceps muscle and tendon, the patella, the patellar tendon, and the tibial tubercle. Disruption of any of these can lead to an extensor mechanism rupture and render an otherwise perfectly good total knee replacement (TKR) useless. Quadriceps tears associated with TKR are difficult to treat and associated with a poorer prognosis than in the native knee. Transosseous sutures tied over the distal pole of the patella are recommended. Patella fractures may heal in continuity or may involve a disruption of the extensor mechanism. The blood supply may have been compromised during TKR, so healing may be delayed. The patella component may be loose and need to be removed or revised. Patellar tendon ruptures are the most common and serious form of extensor mechanism rupture. Prevention is more effective than treatment. Careful exposure during revision surgery includes anticipating the need for a quadriceps snip, pinning the tubercle to avoid avulsion, and preserving the fat pad during primary surgery. Treatment of patellar tendon ruptures is challenging. Primary repair may succeed in early intervention, but in established rupture, allograft reconstruction is often necessary. Achilles tendon allograft is preferred. The calcaneus fragment is embedded into the proximal tibia as a new tubercle, and the tendon is sutured into the remaining extensor mechanism. The repair is then protected using a cable loop from the superior pole of the patella to a drill hole in the upper tibia.

Lateral Retinacular Release in Total Knee Arthroplasty

The indications for, and techniques of, lateral retinacular release are controversial in the orthopedic community. A proposed standard for assessment of patellar tracking before proceeding to a lateral retinacular release is the no-thumb technique. Contributing to the controversy regarding the use of this technique as an indicator for lateral release, studies have shown that differences in tourniquet inflation, surgical approaches, techniques for positioning the femoral, tibial, and patellar components all affect the tests influencing a decision for or against proceeding to lateral release. In addition, the procedure is not without some risk to the patient, even though the specific nature of the complications arising from a lateral release is not entirely clear. Thus, avoiding unnecessary releases are important, though not using them when indicated and having a subluxing and/or dislocating patella is also quite problematic. This article examines the literature for evidence concerning the appropriateness of using lateral retinacular releases and in conjunction with the authors' own experience, makes a recommendation for this procedure to correct patellar maltracking.

Biomechanical Consequences of Patellar Component Medialization in Total Knee Arthroplasty

The optimal amount of patellar component medialization in knee arthroplasty is unknown. We measured the impact, on patellofemoral kinematics and contact force distribution, of 0.0-, 2.5-, and 5.0-mm patellar component medialization in 7 cadaveric specimens implanted with knee arthroplasty components. The knees were flexed dynamically in a weight-bearing rig. Medialization led to lateral shift of the patellar bone, slight medial shift of the patellar component in the femoral groove, lateral tilt of the patella, reduced patellofemoral contact force in later flexion, and lateral shift of the center of pressure in early flexion. Effects on shift and tilt were proportional to the amount of medialization. As a result of this investigation, we recommend medializing the patellar component slightly—on the order of 2.5 mm.

Radiographic features predictive of patellar maltracking during total knee arthroplasty

Despite improvements in component design and surgical technique, some patients still require lateral retinacular release during TKA to improve patella tracking. We studied 148 fixed-bearing TKAs to identify parameters in pre-operative knee radiographs that would predict intraoperative patellar maltracking. Digital radiographs and software were used to measure coronal alignment, distal femoral valgus angle, proximal tibia varus angle, patellar tilt, patellar shift, Insall-Salvati ratio, and patellar component placement and alignment. Patellar tracking was assessed after all components had been cemented, using both no-touch and modified "towel clip" techniques. The only radiographic parameter independently associated with maltracking was patellar shift. The median pre-operative patellar lateral shift in patients who had maltracking was 4.1 mm compared to 0.0 mm in those who did not. Patients who had a patellar shift of more than 3.0 mm had a high likelihood of maltracking, with estimated positive and negative predictive values of 78 and 95%, respectively. Pre-operative patellar shift may thus be clinically relevant for identifying osteoarthritic patients who have a higher likelihood for patellar maltracking during TKA. Variations in the intrinsic risk for maltracking within patient study populations may account for the widely differing reported rates of patellar maltracking, and our data suggest that information on pre-operative patellar shift may be helpful in stratifying these sample populations.

Patellar Management in Revision Total Knee Arthroplasty: Is Patellar Resurfacing a Better Option?

The management of the patella during revision total knee arthroplasty (TKA) depends on the indication for revision, the type and stability of the patellar component in place, and availability of bone stock. We prospectively compared the clinical outcome and satisfaction rates in revision TKA patients managed with patellar resurfacing (n = 13, group I) to retention of the patellar component (n = 22, group II) or patelloplasty (n = 11, group III) at a minimum follow-up of 2 years. There were no differences in the improvement of Knee Society Scores, Short-Form 36 Scores, and satisfaction rates between the groups. There were no revision surgeries for patellar component failure or patellar fractures. Satisfactory results can be achieved using a variety of methods of patellar management in revision TKA by individualizing the treatment modality depending on the clinical scenario.

Dissociation of mobile-bearing patellar component in low contact stress patellofemoral arthroplasty, its mechanism and management: two case reports

IntroductionIsolated patellofemoral osteoarthritis can be treated with patellofemoral arthroplasty.Case presentationWe present two cases of isolated patellofemoral osteoarthritis treated with Low Contact Stress patellofemoral arthroplasty; in both cases the polyethylene mobile-bearing of the patellar component dissociated from the metal backing. One patient had a revision to a Richards patellofemoral prosthesis, and in the second patient the arthroplasty was converted to a total knee prosthesis because of clinically important femorotibial osteoarthritis.ConclusionThe possible failure mechanisms are described. We suggest avoiding the use of a metal-backed mobile-bearing patellar component due to the risk of dissociation.

Ten-year Results of an Inset Biconvex Patella Prosthesis in Primary Knee Arthroplasty

The inset biconvex patella component is an alternative form of patella resurfacing in knee arthroplasty. We retrospectively reviewed 433 patients in whom 521 patella prostheses were implanted before April 1997 to determine survivorship, factors associated with failure of the implant, incidence of anterior knee pain, and factors that may be associated with the latter. We had clinical results for 204 surviving patients (242 knees) without failure of their implants with a minimum 10-year followup (mean, 11.4 years; range, 10-17 years). For the remaining 229 patients we used chart or radiographic review to determine if failure of their implant or other complications had occurred. At latest followup, 14 patella components had been revised for aseptic reasons or were radiographically loose. The 10-year Kaplan-Meier survivorship for the entire cohort for aseptic failure was 97.0%. Aseptic failure of the patella component was associated with the presence of osteonecrosis and the absence of a superior rim of bone radiographically. The incidence of anterior knee pain in surviving patients without failure of their implants was 7.8%. No factor examined was associated with anterior knee pain. Survivorship and clinical and radiographic results are equivalent, but not clearly superior, to those reported for other forms of patella resurfacing. Level IV, therapeutic study.

Magnetic resonance imaging of in vivo patellofemoral kinematics after total knee arthroplasty

Simulated partial weight bearing during magnetic resonance imaging of the knee was used to measure patellar tilt, medial–lateral patellar shift, and patellofemoral contact area in three groups of subjects; patients with posterior cruciate retaining (PCR) TKA, patients with bicruciate substituting (BCS) TKA, and healthy controls. Contact stress was also calculated based on the contact area and body weight-based estimates of contact force. Contact stress was significantly (p<0.05) higher in PCR knees (2.5±3.0 MPa) than in BCS knees (0.2±0.1 MPa) when knees were fully extended, but this difference was not significant (3.7±3.5 MPa for PCR knees vs. 1.4±1.9 MPa for BCS knees; p>0.05) in early flexion. The results also indicate that patellar tilt (normal=2.4°±4.8°, BCS=5.5°±5.5°, PCR=−3.0°±6.9° change in lateral tilt when moving from full extension to early flexion) and contact area (full extension: normal=267±111 mm2, BCS=344±201 mm2, PCR=83±80 mm2; early flexion: normal=723±306 mm2, BCS=417±290 mm2, PCR=246±108 mm2) in BCS TKA mimic those in the normal knees more closely than PCR knees do. These results suggest that the patellar component in BCS TKA may be expected to experience less wear than the patellar component in PCR TKA over time.

A methodology for the pre-clinical evaluation of patellar implants

A methodology for the alignment of patellar and femoral components in pre-clinical test of patellar components used in total knee arthroplasty is presented. The component alignment was based on kinematic degrees of freedom that should be considered to recreate clinically relevant patello-femoral contact patterns. A literature review of patellar kinematics revealed that patellar motion occurs in all six degrees of freedom. Retrieval studies reported that deformation was most prominent on the lateral aspect of the patellar component and in contact regions that occur at higher degrees of knee flexion. Based on this review, it was hypothesised that alignment of the patellar and femoral components should incorporate patellar flexion, proximal-distal translation, anterior-posterior translation and medial-lateral translation. The efficacy of the model was tested by evaluating its ability to create physiological contact patterns in patellar components used in total knee arthroplasty. Two types of components were tested: metal-backed dome and all-polyethylene sombrero-shaped of which the dome-shaped components have a higher reported incidence of failure. The component alignment method resulted in higher contact areas in the sombrero-shaped components than the dome-shaped. Contact patterns of both components were compared to contact patterns reported in the literature for natural and implanted patellae and compared favourably.

Introduction of total knee arthroplasty in Lithuania

Background and purpose We have previously reported that the first 10 years of hip arthroplasty in Lithuania resulted in a higher cumulative revision rate than that observed in Sweden. We thus compared the corresponding results after introduaction of total knee replacement in Lithuania.Methods The 10-year revision rate for the first 595 primary ScanKnee arthroplasties inserted in Klaipeda, Lithuania, was compared to that for the first 1,280 ScanKnee primary arthroplasties inserted in Sweden. As in the hip replacement study, only patients with osteoarthritis (OA) were included. Primary knee arthroplasties without patellar resurfacing were included, and the endpoint was revision for any reason other than addition of a patellar component.Results We found that the cumulative revision rate was not statistically significantly different between the groups. The revision pattern was different, however, and we observed 24 isolated patellar component additions in Sweden, but none in Klaipeda.Interpretation Contrary to the results of our previous hip arthroplasty study, the cumulative revision rate after total knee arthroplasty was similar in the two groups. This suggests that compared to hip arthroplasty, the outcome of total knee arthroplasty was less dependent on surgical experience. The large difference regarding isolated patellar component additions may be explained by long-term accumulation of severe OA cases in Lithuania. To patients subject to a newly introduced surgical treatment offering great improvement in quality of life, patellofemoral pain may be a minor problem. Furthermore, patellar problems may not have seemed particularly relevant for the surgeons, considering the disability of other patients waiting to be treated.

Specific Complications of the Mobile-Bearing Total Knee Prosthesis

Total knee arthroplasty (TKA) is considered as a standard procedure for the treatment of the severe degenerative knee joint. The outcomes of TKA have improved over the last few decades because of a better understanding of design features and surgical techniques. Contemporary tibial component designs in TKA can be subdivided into two groups (fixed or mobile-bearing) on the basis of different fundamental principles. The mobile-bearing knee is designed with better biomechanical concepts than the fixed-bearing knee, in terms of providing congruity and mobility of tibial bearing. It is expected that the mobile-bearing knee could resolve the problems of component wear and component loosening as seen in fixed-bearing knees. However, the long-term durability of mobile-bearing knees has not been extensively proven to have better outcomes than that of fixed-bearing knees. There are some potential clinical complications of mobile-bearing knees that must be addressed. On the basis of our previous clinical experience and a retrieval analysis, this article summarizes five specific complications including (1) early or late dislocation of the mobile-bearing component, (2) failure or dislodging of the meniscal bearing, (3) hemarthrosis associated with a failure of the meniscal-bearing knee, (4) failure of the patellar component, and (5) higher prevalence of osteolysis. Before taking advantage of mobile bearing, we should consider these specific clinical complications, which were not found in the fixed-bearing TKA.

The cemented inset biconvex patella in revision knee arthroplasty

Evaluation of a cemented biconvex inset patellar component used in revision knee arthroplasty at minimum five year follow-up was undertaken. Of the initial cohort of 89 knees in 85 patients, two patellar implants were revised for aseptic loosening following a transverse fracture of the patella associated with avascular necrosis. A further four implants were judged radiographically loose. Aseptic loosening of the implant was strongly correlated with the presence of avascular necrosis radiographically. Fracture of the patellar bone remnant was associated with a radiographically measured thickness of residual patellar bone of less than 6 mm. Survivorship of the implant using aseptic revision as the endpoint was 98% at 10 years and 86% at 14 years given one late failure. We conclude that the cemented biconvex inset patellar component can give satisfactory results in revision of patellar components if avascular necrosis does not occur.

Resection arthroplasty for failed patellar components

A total of 1,401 primary total knee arthroplasties (TKA) were reviewed; 44 (3.2%) had at least the patellar component revised. Nine of these knees (eight patients) had insufficient bone stock to allow reimplantation of another patellar component. Clinical data on the nine knees were obtained with recent follow-up evaluation, review of their medical records and radiographs. Evaluation included Hospital for Special Surgery (HSS) scores. Average follow-up was 4 years and 7 months, 2-year range (2 months to 8 years and 4 months). Common factors found in these nine knees included: thin patella after primary TKR status, osteoarthritis, good range of motion and patella alta. Results were good to excellent in seven knees and fair in two. The untoward associations with patellectomy such as quadriceps lag, extension weakness and anterior knee pain were not experienced. Resection of the patellar component, without reimplantation, is an acceptable alternative in revision TKA lacking adequate remaining bone stock.

Trabecular metal patella in total knee arthroplasty with patella bone deficiency

This study evaluates results following patellar resurfacing using trabecular metal (TM) patella in marked deficiency or weakness of patellar bone that precludes patellar resurfacing with a standard cemented patellar button. Ten consecutive patients undergoing primary (3 cases) or revision (7 cases) total knee arthroplasty with patella augmentation were evaluated at a mean follow-up of 45 months (range 18–65). Nine patients had marked patellar bone deficiency and one had had previous patellectomy. No intra-operative complications occurred. There was no displacement of the patellar component and no patellar fractures when at least 50% of bone contact was possible. We observed loosening of the patella augmentation 17 months after the index procedure only in the case of previous patellectomy. When bone was present the fixation appeared excellent by radiographic evaluation already at 3 to 6 months after surgery; afterward bone contact was uniform in the peripheral regions in both lateral and Merchant radiographic views without signs of loosening. Finally, the mean Knee Society scores improved in all patients.

Effect of an UHMWPE patellar component on stress fields in the patella: a finite element analysis

An increased stress in the patella due to the implantation of a patellar button may also be another potential source of pain in total knee arthroplasty patients. This study assessed the location inside the patella having largest stress change after implantation of an ultra high molecular polyethylene patella button. Finite elements models of the patellae before and after implantation of patellar button were created. Experimentally determined spring constants of muscles and ligaments, and patellofemoral contacting loads were applied to the models at 30 degrees , 60 degrees , and 90 degrees of knee flexion. The Von Mises stress of the intact patella decreased with increased knee flexion, while that of implanted patella increased. Also, the stress range in the implanted patella was 3-9 times higher than in the intact one. The highly stressed region of the intact patella moved proximally with higher knee flexion angles, while that of the implanted model stayed near the central anterior patella. At 90 degrees of knee flexion, the stress in the anterodistal patella increased considerably after implantation of a patella button so that the anterodistal patella may be susceptible to be painful source after the total knee replacement.

Management of the Deficient Patella in Revision Total Knee Arthroplasty

There are a number of options available to manage the patella when revising a failed total knee arthroplasty. If the previous patellar component is well-fixed, undamaged, not worn, and compatible with the femoral revision component, then it can be retained. When a patellar component necessitates revision and is removed with adequate remaining patellar bone stock, an onlay-type all-polyethylene cemented implant can be used. Management of the patella with severe bony deficiency remains controversial. Treatment options for the severely deficient patella include the use of a cemented all-polyethylene biconvex patellar prosthesis, patellar bone grafting and augmentation, patellar resection arthroplasty (patelloplasty), performing a gull-wing osteotomy, patellectomy, or the use of newer technology such as a tantalum (trabecular metal) patellar prosthesis. Severe patellar bone deficiency is a challenging situation because restoration of the extensor mechanism, proper patellar tracking, and satisfactory anatomic relationships with the femoral and tibial components are critical for an optimal clinical outcome. Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Predicting Patellar Failure After Total Knee Arthroplasty

We identified patient and surgeon factors associated with patellar component failure in a series of 8530 TKAs performed in 5640 patients using the same posterior cruciate ligament-retaining TKA with all-polyethylene patellar components between January 1983 and December 2003. Patellar failure was defined as loosening, fracture, or patellar revision. All infections were excluded. Statistical analysis using Kaplan-Meier and Cox regression was used to determine the risk of patellar failure. Followup averaged 7.0 years (range, 2-22 years). Patellar component loosening occurred in 4.8% of TKAs (409 knees). Patellar fracture was identified in 5.2% of TKA (444 knees). Twenty-five patellae were revised (0.3%). TKA performed with a lateral release and patients with a body mass index of greater than 30 kg/m(2) were at the greatest risk of patellar loosening and fracture, respectively. Male gender, preoperative varus alignment of greater than 5 degrees , and large patellar component size also predicted a higher risk of patellar fracture. Medial patellar component position, tibial component thickness of greater than 12 mm, preoperative valgus alignment of 10 degrees or more, and preoperative flexion of 100 degrees or more predicted patellar loosening. An awareness of these factors that predict patellar failure after TKA may help determine the relative indications for TKA and influence surgical technique, especially when using this prosthesis. Level IV, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Isolated All-polyethylene Patellar Revisions for Metal-backed Patellar Failure

The outcome of isolated patellar component revisions after metal-backed patellar failure is variable with satisfactory results reported from 78% to 100%. To supplement information in the literature we determined the failure rate and the functional outcome based on the Knee Society clinical and roentgenographic evaluation systems of isolated patellar component revisions after metal-backed patellar component failure. We retrospectively reviewed 27 patients with 28 isolated patellar component revisions for metal-backed patellar component failure performed between 1988 and 2005. Twenty-five knees in 24 patients were available for review with a minimum followup of 24 months (mean, 90 months; range, 24-210 months). All knees were revised with a cemented all-polyethylene patellar component and all tibial polyethylene components were routinely exchanged. One failure (4%) occurred 122.6 months after the isolated patellar component revision secondary to femoral and tibial component loosening. The average Knee Society knee score improved from 73 to 89 points, whereas the average Knee Society function score improved from 56 to 65. Our data confirm those in the literature suggesting a successful outcome can be achieved with an isolated patellar component revision for metal-backed patellar component failure. Level IV, retrospective case series. See the Guidelines for Authors for a complete description of levels of evidence.