Rigidity of initial fixation with uncemented tibial knee implants

This study quantifies the in vitro motion occurring between bone and cemented and noncemented tibial components. Liquid metal strain gauges were used to measure the motion between the tibial component and bone at four locations in eight cadaver tibia at near-point cyclic loads ranging from 10 to 2,000 N. Two types of motion were observed: inducible displacement, which is reversible, followed the oscillating load and occurred in both cemented and uncemented tests, and liftoff or separation of the component and bone, which occurred only for the noncemented cases and remained even after removal of the load. For both motion types, noncemented tests exhibited significant (P less than .05) and dramatic increased interface motion compared to the cemented cases for all load types. The results suggest that the magnitudes of implant-bone interface separation at loads in the low physiologic range for noncemented implants can be sufficiently large to inhibit bony ingrowth into a prosthesis with an average pore size of 300 microns.     

Early postoperative fixation of tibial components: An in vivo roentgen stereophotogrammetric analysis

The fixation of cemented and noncemented tibial components in 19 total knee arthroplasties was examined 3 to 10 weeks postoperatively with roentgen stereophotogrammetric analysis. Physiologic external forces were applied either in outward-inward rotation or as eccentric posterior loading generated by squatting. In one case there was no motion, while in the remaining 18 cases relative interface motion of up to 1 3 mm was found. In some cases, the largest motion was found during inward-outward rotatory stress, while in others, the largest motion was found during squatting. Posterior tilt was weakly correlated with posterior displacement of the femoral component on the tibia during squatting (r² = 0.323, p < 0.05). For the noncemented cases, the initial fixation to bone was probably insufficient, and ingrowth of bone would not have been achieved. For the cemented cases, motion of the implant was reduced. Given the short postoperative time and the probable absence of any substantial fibrous tissue membrane, we suggest that the observed motion represented elastic deformation of the bone.     

Biomechanical study of pedicle screw fixation in severely osteoporotic bone.

BACKGROUND CONTEXT: Obtaining adequate purchase with standard pedicle screw techniques remains a challenge in poor quality bone. The development of alternate insertion techniques and screw designs was prompted by recognition of potential fixation complications. An expandable pedicle screw design has been shown to significantly improve fixation compared to a conventional screw in poor quality bone. PURPOSE: The purpose of this study was to determine if polymethylmethacrylate (PMMA) bone cement augmentation of an expandable pedicle screw can further improve fixation strength compared to the expandable screw alone in severely osteoporotic bone. A technique for cement insertion into the pedicle by means of the cannulated central portion of the expandable screw is also described. STUDY DESIGN: The axial pullout strength, stiffness and energy absorbed of cemented and noncemented expandable pedicle screws was determined in cadaveric vertebrae. METHODS: Twenty-one fresh unembalmed vertebrae from the thoracolumbar spine were used. Radiographs and bone mineral density measurements (BMD) were used to characterize bone quality. Paired cemented and noncemented pedicle screw axial pullout strength was determined through mechanical testing. Mechanical pullout strength, stiffness and energy to failure was correlated with BMD. RESULTS: Overall, there was a 250% increase in mean pullout strength with the cemented expandable screw compared with a noncemented expandable screw including a greater than twofold increase in pullout strength in the most severely osteoporotic bone. The mean stiffness and energy absorbed to failure was also significantly increased. A cemented conventional screw achieved a pullout strength similar to the noncemented expandable screw. CONCLUSIONS: PMMA cement augmentation of the expandable pedicle screw may be a viable clinical option for achieving fixation in severely osteoporotic bone.     

Results of 1,000 performance knees: Cementless versus cemented fixation

A total of 893 patients with 1,000 Performance total knee prostheses were retrospectively studied. The mean follow-up was 5.2 years. In 584 cases, the femoral and tibial components were implanted using cementless techniques, and in 416 knees the femoral and tibial components were cemented. All patients received a cemented all polyethylene patellar replacement. Tibial bone density determined fixation type. The average age of patients with cementless fixation was 64.3 years versus 76.2 years for patients with cemented implants. The average subjective and functional Knee Society scores were 91.2 and 90.1 for patients with cementless knees and 89.6 and 83.5 for those with cemented replacements. A surprising absence of osteolysis around screw fixation was noted, and at 5 years, there was 99% implant survival.     

The mechanism of loosening of cemented acetabular components in total hip arthroplasty. Analysis of specimens retrieved at autopsy.

Late aseptic loosening of cemented acetabular components is governed by the progressive, three-dimensional resorption of the bone immediately adjacent to the cement mantle. This process begins circumferentially at the intraarticular margin and progresses toward the dome of the implant. Evidence of bone resorption at the cement-bone interface was present even in the most well-fixed implants before the appearance of lucent lines on standard roentgenographic views. The mechanical stability of the implant was determined by the three-dimensional extent of bone resorption and membrane formation at the cement-bone interface. The leading edge of the membrane is a transition zone from regions of membrane interposition between the cement and the bone to regions of intimate cement-bone contact. Histologic analysis revealed that progressive bone resorption is fueled by small particles of high density polyethylene (HDP) migrating along the cement-bone interface and bone resorption occurs as a result of the macrophage inflammatory response to the particulate HDP. Evidence in support of a mechanical basis for failure of fixation was lacking. The mechanism of late aseptic loosening of a cemented acetabular component is therefore biologic in nature, not mechanical. This is exactly opposite to the mechanism of loosening on the femoral side of a cemented total hip replacement, which is mechanical in nature.     

Biomechanical stability of intramedullary nailed high proximal third tibial fractures with cement augmented proximal screws

OBJECTIVE: Intramedullary nailing of nonarticular proximal tibia fractures can be affected by bone density resulting in loss of stability, fixation, and malalignment in osteopenic bone. This study was designed to quantify the biomechanical effects of augmenting proximal screws with cement in intramedullary nailing of high proximal third tibial fractures. DESIGN: In vitro biomechanical study using anatomic specimens. METHODS: Reamed nails were inserted into seven pairs of fresh-frozen cadaveric proximal tibiae and secured using two oblique and two transverse proximal screws. Paired tibiae were randomly assigned into two groups: cemented and noncemented proximal screw-holes. Bone cement was injected into the screw-holes before screw insertion in the cemented tibiae. Specimens were then tested in flexion/extension and varus/valgus to 12 Nm and in torsion to 7 Nm. Physical measurements of bone density were obtained to determine the effect of density on stability. MAIN OUTCOME MEASURES: Stability of the construct in both groups was analyzed and compared statistically using paired t tests. RESULTS: Cement augmentation of the proximal screws significantly increased mechanical stability in torsion and varus/valgus load configurations, with average decreases in rotational motion of 5.4 degrees +/- 1.6 degrees and 5.1 degrees +/- 5 degrees respectively. No change in stability was observed in flexion/extension loading. A trend toward decreased stability was seen in the uncemented construct in varus/valgus; cement augmentation of the proximal screws eliminated this effect. CONCLUSIONS: Lower bone density decreased the stability of the uncemented construct; however, cement augmentation of the proximal screws showed a trend to eliminate this effect in the varus/valgus loading configuration and should be considered when nailing proximal third tibial fractures in osteoporotic patients.     

Mechanical stability of cementless tibial components in normal and osteoporotic bone

There is renewed interest in cementless total knee arthroplasty, yet the optimal tibial design remains elusive. The purpose of this study is to evaluate the stability of various tibial component designs in normal and osteoporotic bone. Two cementless tibial implants, a CoCr beaded keeled baseplate and a two-pegged, highly porous titanium design underwent mechanical testing in normal and osteoporotic bone models. Decreased stability was observed in both designs in the osteoporotic model compared with "normal" bone (p < 0.0017). Stability of the keeled design was greater than the two-peg design in both densities; however, this difference was statistically significant only in the "normal" bone model (p = 0.005). This study demonstrates that cementless tibial component stability is affected by both implant design and host bone quality.     

Tibial component fixation with cement: full- versus surface-cementation techniques

Despite excellent outcomes with cemented tibial components in total knee arthroplasty, it still is debated whether the tibial stem should be cemented and what the optimal tibial stem design should be. Proponents of full cementation of the tibial stem and component state that better short-term and long-term component fixation is achieved when full cementation is used. Advocates for surface cementation contend that sufficient implant stability is achieved without the increased bone loss that occurs at revision and the stress shielding thought to be linked with cemented stems. This biomechanical cadaver study compared initial fixation and cement penetration depth in fully cemented versus surface cemented tibial trays with two different stem geometries (cruciate and I-beam) and compared two stem designs (cruciate and I-beam) fixed with surface cementation. Under an eccentric load, simulating three times body weight for 6000 cycles, there seems to be no difference in the micromotion of either tibial component implanted with surface or full cementation. Additionally, no difference in the average depth of cement penetration was detected between fixation techniques or stem types. The initial fixation stability of the surface cement technique seems correlated to the depth of cement penetration into proximal tibial surface. The current data support other studies which indicate that stability of surface-cemented tibial components may be related to the depth of cement penetration.     

Results of noncemented revision

The results of revision of total hip replacements with noncemented fixation are dependent on implant design, postoperative treatment, and the technique selected in relationship to the bone quality and quantity. When proximal femoral bone ingrowth fixation is desired an implant must have a double wedge design, which permits a high percentage contact between the host bone and the porous coating of the implant. Bone graft in either the femur or acetabulum should be used at a minimum. Every attempt to obtain host bone contact to porous coating should be done. When a bone graft is used to replace the proximal femur or when an acetabular bone graft covers more than 50% of the cup, the implant should be cemented into the bone graft. Bone structure affects the outcome of noncemented revisions more than the thickness of the bone. Even thin and weak femoral bone has dynamic remodeling capability. The structure influences result more according to the ability to contact the host bone to the porous coating. When this can be done, the bone will heal to the porous coating. Postoperatively, 40% of our patients are treated with a cast to protect bone grafts and weak bones that are important for stability of the implant. We believe that conservative postoperative rehabilitation for noncemented revision arthroplasty significantly improves the healing for the bone to the implant.     

Assessment of Total Hip Arthroplasty by Means of Computed Tomography 3D Models and Fracture Risk Evaluation

Total hip arthroplasty (THA) can be achieved by using a cemented or noncemented prosthesis. Besides patient's age, weight, and other clinical signs, the evaluation of the quality of the bones is a crucial parameter on which orthopedic surgeons base the choice between cemented and noncemented THA. Although bone density generally decreases with age and a cemented THA is preferred for older subjects, the bone quality of a particular patient should be quantitatively evaluated. This study proposes a new method to quantitatively measure bone density and fracture risk by using 3D models extracted by a preoperative computed tomography (CT) scan of the patient. Also, the anatomical structure and compactness of the quadriceps muscle is computed to provide a more complete view. A spatial reconstruction of the tissues is obtained by means of CT image processing, then a detailed 3D model of bone mineral density of the femur is provided by including quantitative CT density information (CT must be precalibrated). A finite element analysis will provide a map of the strains around the proximal femur socket when solicited by typical stresses caused by an implant. The risk for structural failure due to press‐fitting and compressive stress during noncemented THA surgery was estimated by calculating a bone fracture risk index (ratio between actual compressive stress and estimated failure stress of the bone). A clinical trial was carried out including 36 volunteer patients (ages 22–77) who underwent unilateral THA surgery for the first time: 18 received a cemented implant and 18 received a noncemented implant. CT scans were acquired before surgery, immediately after, and after 12 months. Bone and quadriceps density results were higher in the healthy leg in about 80% of the cases. Bone and quadriceps density generally decrease with age but mineral density may vary significantly between patients. Preliminary results indicate the highest fracture risk at the calcar and the lowest at the intertrocanteric line, with some difference between patients. An analysis of the results suggest that this methodology can be a valid noninvasive decision support tool for THA planning; however, further analyses are needed to tune the technique and to allow clinical applications. Combination with gait analysis data is planned.     

Continued stabilization of trabecular metal tibial monoblock total knee arthroplasty components at 5 years-measured with radiostereometric analysis

Background and purpose

The trabecular metal tibial monoblock component (TM) is a relatively new option available for total knee arthroplasty. We have previously reported a large degree of early migration of the trabecular metal component in a subset of patients. These implants all appeared to stabilize at 2 years. We now present 5-year RSA results of the TM and compare them with those of the NexGen Option Stemmed cemented tibial component (Zimmer, Warsaw IN).

Patients and methods

70 patients with osteoarthritis were randomized to receive either the TM implant or the cemented component. RSA examination was done postoperatively and at 6 months, 1 year, 2 years, and 5 years. RSA outcomes were translations, rotations, and maximum total point motion (MTPM) of the components. MTPM values were used to classify implants as “at risk” or “stable”.

Results

At the 5-year follow-up, 45 patients were available for analysis. There were 27 in the TM group and 18 in the cemented group. MTPM values were similar in the 2 groups (p = 0.9). The TM components had significantly greater subsidence than the cemented components (p = 0.001). The proportion of “at risk” components at 5 years was 2 of 18 in the cemented group and 0 of 27 in the TM group (p = 0.2).

Interpretation

In the previous 2-year report, we expressed our uncertainty concerning the long-term stability of the TM implant due to the high initial migration seen in some cases. Here, we report stability of this implant up to 5 years in all cases. The implant appears to achieve solid fixation despite high levels of migration initially.Swedish Knee Arthoplasty Register data suggest that uncemented tibial components have worse survivorship than cemented components (Knutson and Robertsson 2010). However, more recent data from Australia have suggested that modern uncemented designs have a similar cumulative revision rate to that of cemented components (Graves et al. 2004). With recent improvements in biomaterials, the potential for improved longevity using uncemented implants is once again being explored. Trabecular metal (Zimmer, Warsaw, IN) is a new material available for use in uncemented total knee arthroplasty. It is a porous biomaterial with morphology and mechanical properties resembling those of trabecular bone (Bobyn et al. 1999a,b, Zardiackas et al. 2001, Levine et al. 2006, Balla et al. 2010). In a previous paper, we presented the 2-year implant migration results of the trabecular metal tibial monoblock component using radiostereometric analysis (RSA) (Dunbar et al. 2009). The results of that study showed high initial migration in 9 out of 28 of the tibial components, with apparent stabilization occurring after 1 year in all cases. These results reflected the results of a similar RSA study on the same implant, performed at another center (Henricson et al. 2008). In addition, in the 2-year report we also documented apparent deformation of the tibial base plate, which occurred in 5 of the cases of high migration (Dunbar et al. 2009). The implications of the high degree of migration and plate deformation for long-term survival are not known. In a subset of the patients enrolled in the study, we also performed RSA for measurement of inducible displacement between 2 and 4 years. The results of this study showed that the trabecular metal component had the lowest inducible displacement ever reported in the literature, indicating excellent stability (Wilson et al. 2010). The results of these 3 studies supported the hypothesis that these components were achieving adequate bone in-growth for long-term survival (Henricson et al. 2008, Dunbar et al. 2009, Wilson et al. 2010). However, longer follow-up is necessary to determine whether the early stability of these implants is durable. In this paper, we present the 5-year longitudinal RSA results from the original cohort of patients randomized to receive either the Nexgen LPS monoblock (trabecular metal) tibial component (Zimmer) or the cemented NexGen Option Stemmed tibial component (Zimmer) (Dunbar et al. 2009).     

Implant stability is affected by local bone microstructural quality

It is known that low bone quality, caused for instance by osteoporosis, not only increases the risk of fractures, but also decreases the performance of fracture implants; yet the specific mechanisms behind this phenomenon are still largely unknown. We hypothesized that especially peri-implant bone microstructure affects implant stability in trabecular bone, to a greater degree than more distant bone. To test this hypothesis we performed a computational study on implant stability in trabecular bone. Twelve humeral heads were measured using micro-computed tomography. Screws were inserted digitally into these heads at 25 positions. In addition, at each screw location, a virtual biopsy was taken. Bone structural quality was quantified by morphometric parameters. The stiffness of the 300 screw-bone constructs was quantified as a measure of implant stability. Global bone density correlated moderately with screw-bone stiffness (r2=0.52), whereas local bone density was a very good predictor (r2=0.91). The best correlation with screw-bone stiffness was found for local bone apparent Young's modulus (r2=0.97), revealing that not only bone mass but also its arrangement in the trabecular microarchitecture are important for implant stability. In conclusion, we confirmed our hypothesis that implant stability is affected by the microstructural bone quality of the trabecular bone in the direct vicinity of the implant. Local bone density was the best single morphometric predictor of implant stability. The best predictability was provided by the mechanical competence of the peri-implant bone. A clinical implication of this work is that apparently good bone stock, such as assessed by DXA, does not guarantee good local bone quality, and hence does not guarantee good implant stability. New tools that could quantify the structural or mechanical quality of the peri-implant bone may help improve the surgical intervention in reaching better clinical outcomes for screw fixation.     

Proximal Tibial Bone Density Is Preserved After Unicompartmental Knee Arthroplasty

Background

Bone mineral density (BMD) in the proximal tibia decreases after TKA and is believed to be a factor in implant migration and loosening. Unicompartmental knee arthroplasty (UKA) is a less invasive procedure preserving knee compartments unaffected by degeneration. Finite element studies have suggested UKA may preserve BMD and that implants of differing stiffnesses might differentially affect BMD but these notions have not been clinically confirmed.

Questions/purposes

We therefore asked whether (1) proximal tibial BMD decreases after UKA, and (2) a cemented metal tibial component with a mobile polyethylene (PE) bearing would have greater BMD loss than a cemented PE tibial component.

Methods

We prospectively followed 48 patients who underwent 50 UKAs using one of two implants: one with a cemented metal tibial baseplate and a mobile PE insert (n = 26) and one with a cemented all-PE tibial component (n = 24). In followup we assessed pain and function (Oxford Knee Score, SF-12, The Knee Society Score^©) and radiographs. BMD changes were assessed using quantitative CT osteodensitometry performed postoperatively and at 1 and 2 years after the index procedure.

Results

Mean cancellous BMD decreased 1.9% on the medial side and 1.1% on the lateral side. Mean cortical BMD was static, decreasing 0.4% on the medial side and increasing 0.5% on the lateral side. The greatest observed difference between implants for any region was 3.7%. There were no differences in pain or functional outcome scores.

Conclusions

BMD was preserved 2 years after UKA with no major differences seen between implant types.     

Primary total hip arthroplasty in patients with rheumatoid arthritis

Twenty-eight (11 cemented and 17 noncemented) total hip arthroplasties (THA) were performed in 20 patients with rheumatoid arthritis (RA). The average age at operation was 42.1 years and the average follow-up was 10.8 years. There were two deep infections requiring removal of the prosthesis. Three cemented acetabular cups and one cemented femoral component were revised due to aseptic loosening. One cemented cup was loosened radiologically. One PCA polyethylene liner was revised because of significant wear. Both cemented and noncemented femoral components are capable of providing respectable results in RA patients. The relatively inferior results of THA among RA patients is due not only to the fixation method, but also to the poorer bone quality.     

A new model to assess tibial fixation. II. Concurrent histologic and biomechanical observations

A tibial hemiarthroplasty model was designed to allow concurrent histologic and biomechanical analysis of the tibial implant-bone interface. In this study, micromotion was visually observed at the implant-bone interface of cemented and uncemented implants. Six dogs had staged bilateral implantation. Biomechanical analysis of three- and 12-month specimens was correlated with histologic analysis of the same specimen. Load transmission involved compression of trabecular bone and fibrous tissue at the interface. Failure of the interface occurred through cyclic fatigue and microfracture of trabeculae. Micromotion was seen at all interfaces, porous or smooth, cemented or uncemented. Displacement was greatest under an eccentrically loaded plateau. Micromotion at ingrown and cemented interfaces was because of trabecular compression. Uncemented smooth devices and uncemented porous devices with fibrous fixation of the plateau appeared unstable. The cemented implants and a single uncemented implant studied at 12 months showed no interface micromovement. The absence of micromovement in the uncemented implant was associated with subsidence and with ingrowth of 30% into the porous peg and porous plateau.     

Fixation of proximal tibia fractures by a retrograde nail: a biomechanical investigation

Surgical treatment of proximal tibial fractures requires open reduction and internal fixation. The operative exposure causes additional soft-tissue injury and reduces the blood supply to the bone. A cephalograde tibial nail should offer comparable mechanical stability without these disadvantages. We compared the stability of both osteosyntheses in a fracture model with 12 fresh-frozen cadaver bones. While both implants exhibited comparable stiffness under sagittal loading, the plate had a higher rotational and varus stiffness. Despite this higher stiffness, rotational displacements at the fracture gap were nearly twice as large for this implant during loading. We conclude that the retrograde nail provides similar mechanical stability to plate fixation for proximal tibial fractures, while the closed reduction and soft-tissue preservation of this new technique are definite advantages. Received: 28 July 1998     

Tibial Implant Fixation Behavior in Total Knee Arthroplasty: A Study With Five Different Bone Cements

BackgroundThe objectives of this study are to (1) evaluate if there is a potential difference in cemented implant fixation strength between tibial components made out of cobalt-chromium (CrCoMo) and of a ceramic zirconium nitride (ZrN) multilayer coating and to (2) test their behavior with 5 different bone cements in a standardized in vitro model for testing of the implant-cement-bone interface conditions. We also analyzed (3) whether initial fixation strength is a function of timing of the cement apposition and component implantation by an early, mid-term, and late usage within the cement-specific processing window.MethodsAn in vitro study using a synthetic polyurethane foam model was performed to investigate the implant fixation strength after cementation of tibial components by a push-out test. A total of 20 groups (n = 5 each) was used: Vega PS CrCoMo tibia and Vega PS ZrN tibia with the bone cements BonOs R, SmartSet HV, Cobalt HV, Palacos R, and Surgical Simplex P, respectively, using mid-term cement apposition. Three different cement apposition times—early, mid-term, and late usage—were tested with a total of 12 groups (n = 5 each) with the bone cements BonOs R and SmartSet HV.ResultsThere was no significant difference in implant-cement-bone fixation strength between CrCoMo and ZrN multilayer-coated Vega tibial trays tested with 5 different commonly used bone cements.ConclusionApposition of bone cements and tibial tray implantation in the early to mid of the cement-specific processing window is beneficial in regard to interface fixation in TKA.     

Inducible Displacement of a Trabecular Metal Tibial Monoblock Component

Radiostereometric analysis is a highly accurate technique that can be used for measuring micromotion at the bone-implant interface. The purpose of this study was to compare the inducible displacement of the uncemented Trabecular Metal (TM; Zimmer, Warsaw, Ind) tibial monoblock component with that of a cemented implant. Inducible displacement of 14 uncemented TM components and 11 cemented components was measured 24 to 48 months postoperatively. Longitudinal migration of the implants was also measured with radiostereometric analysis at 6, 12, and 24 months postoperatively. The uncemented TM group had significantly lower inducible displacement than the cemented components. Significant correlations were found between longitudinal migration and the inducible displacement tests. The low values of inducible displacement in the TM group indicated good fixation and a promising long-term prognosis.     

Mechanical Stability of Well-Functioning Tibial Baseplates From Postmortem-Retrieved Total Knee Arthroplasties

In joint replacement, cyclic motion at the bone-prosthesis interface is considered a precursor to component loosening. This study characterized the mechanical stability of 13 total knee arthroplasties harvested postmortem after an average time in situ of 10.3 years. With loads applied to the medial and then the lateral tibial plateau, motion between the tibial component and underlying bone was measured with extensometers. The amount of motion between the tibial component and underlying bone under medial and lateral loads of 500 N and then twice body weight was typically less than 20 μm. Tray depression under load application and the liftoff on the contralateral side indicated that the tibial stems limited implant rotation and that implant fixation did not deteriorate with time in situ.     

Superior fixation of pegged trabecular metal over screw-fixed pegged porous titanium fiber mesh: a randomized clinical RSA study on cementless tibial components

Background and purpose

Lasting stability of cementless implants depends on osseointegration into the implant surface, and long-term implant fixation can be predicted using radiostereometric analysis (RSA) with short-term follow-up. We hypothesized that there would be improved fixation of high-porosity trabecular metal (TM) tibial components compared to low-porosity titanium pegged porous fiber-metal (Ti) polyethylene metal backings.

Methods

In a prospective, parallel-group, randomized unblinded clinical trial, we compared cementless tibial components in patients aged 70 years and younger with osteoarthritis. The pre-study sample size calculation was 22 patients per group. 25 TM tibial components were fixed press-fit by 2 hexagonal pegs (TM group) and 25 Ti tibial components were fixed press-fit and by 4 supplemental screws (Ti group). Stereo radiographs for evaluation of absolute component migration (primary effect size) and single-direction absolute component migration (secondary effect size) were obtained within the first postoperative week and at 6 weeks, 6 months, 1 year, and 2 years. American Knee Society score was used for clinical assessment preoperatively, and at 1 and 2 years.

Results

There were no intraoperative complications, and no postoperative infections or revisions. All patients had improved function and regained full extension. All tibial components migrated initially. Most migration of the TM components (n = 24) occurred within the first 3 months after surgery whereas migration of the Ti components (n = 22) appeared to stabilize first after 1 year. The TM components migrated less than the Ti components at 1 year (p = 0.01) and 2 years (p = 0.004).

Interpretation

We conclude that the mechanical fixation of TM tibial components is superior to that of screw-fixed Ti tibial components. We expect long-term implant survival to be better with the TM tibial component.Tibial component loosening remains one of the major causes of failure of cementless total knee arthroplasty (TKA), and the early degree of knee implant migration detected by radiostereometric analysis (RSA) has been shown to predict the long-term survival of the implant (Ryd et al. 1995). With cementless knee arthroplasty, stability is achieved by biological fixation within the first weeks after surgery and the success relies on both correct component position and immediate macrofixation (Soballe et al. 1992). Porous implant surfaces support tissue ingrowth and are generally effective in supplementing bony integration of cementless implants (Bobyn et al. 1982). On the other hand, fibrous integration of tibial knee components leads to reduced strength of mechanical fixation, which is detectable under physiological loads, and to increased early migration measured with RSA, and it may indicate an increased risk of loosening at a later stage (Bellemans 1999). RSA is therefore particularly useful during the first postoperative years (Valstar et al. 2005).The pore size and structural geometry of coatings in cementless arthroplasty are important factors for early and safe bone ingrowth. Low-porosity coatings, i.e. fiber-metals and beads, may have inferior osseointegration compared to high-porosity coatings with regular interconnecting pores, i.e. trabecular metal (tantalum), which is a newer prosthetic material (Bobyn et al. 1982, Bobyn 1999).The prosthetic design also influences implant survival and function. A monobloc tibial design offers advantages compared to a modular design in terms of elimination of back-side wear problems and elimination of metallic debris produced by the polyethylene locking mechanism. A pegged tibial design without screw-holes provides an increased surface area for bony fixation and eliminates points where wear debris can directly enter the bone. On the other hand, a modular and screw-fixed design offers a consistent intraoperative macro-fixation (Sumner et al. 1994) with the option of isolated polyethylene liner revision later on (Ryd et al. 1993).It has been recommended that the fixation of new products for prosthetic surgery should be evaluated by RSA prior to general use (Valstar et al. 2005), and at the time of initiation of this study no clinical RSA data were available for the trabecular metal implant. The aim of this randomized clinical trial (RCT) was to compare the early clinical and migration results (absolute total migration and absolute single-direction migration) in younger osteoarthritic patients treated with two different cementless tibial implants: a new double-pegged trabecular metal tibial component and a well-documented porous, pegged screw-fixed titanium fiber-mesh tibial component.