In this retrospective study, we evaluated the correlation between extension stem characteristics and prosthesis stability following RTKA with the same type of prosthesis (Zimmer® LCCK) and fixation technique (hybrid fixation) in patients with minor bone defect (AORI type I/IIa). We tried to set up a principle of stem choice, by which surgeons can estimate the optimal CFR and stem length in preoperative templating or intraoperative trial. We found that the optimal choice of extension stem was CFR0.85 > 0 cm or CFR0.7 > 2 cm for the femoral component, and CFR0.85 > 0 cm for the tibial component. Male sex and bone defect of AORI type IIa were associated with femoral component loosening, while varus alignment was associated with tibial component loosening.
To date, there are no clear guidelines regarding the choice of extension stem in RTKA. Parsley et al. reported better alignment for CFR > 0.85 [5], while Gililland et al. reported good stability for diaphyseal engagement > 4 cm without defining diaphyseal engagement or specifying the reason behind choosing this cutoff [6]. To our knowledge, the present study is the first to evaluate the potential association of RTKA stability with extension stem diameter and length. The various CFR-based criteria evaluated here were identified based on ROC curve analysis, which indicated CFR > 0.72 and CFR > 0.85 as predictors of prosthesis loosening on the femoral and tibial side, respectively. We found that not only the diameter of the stem, but also its length can be important for prosthesis stability. Indeed, CFR0.7 > 2 cm and CFR0.7 > 4.3 cm were also predictors of femoral and tibial component stability, respectively.
Our results suggest that CFR0.7 > 2 cm is sufficient to ensure femoral component stability, while tibial component stability requires CFR > 0.85, which might imply that the tibial component is more prone to loosening. Previous observations in this respect are highly discrepant. Leta et al. reported that loosening was almost twice more likely to occur at the tibial than at the femoral side [2]. Fehring et al. reported higher loosening rate at the femoral side, while others found similar loosening rates for the femoral and tibial components [6, 8, 9]. The discrepancy in these findings might be related to bone defect severity. Specifically, we only included knees with minor defects (AORI type I/IIa), which likely provided relatively good bone-prosthesis contact in the femoral box area, resulting in improved stability. In addition, the femoral component is intrinsically more stable than the tibial component because of the “pinching” effect provided by the anterior flange and posterior condyle of the femoral prosthesis, which is known to affect the stability of cementless TKA [10, 11]. Previous studies on primary TKA also reported more aseptic loosening at the tibial than at the femoral side [12, 13]. Thus, the femoral stem may not need a very high CFR to achieve good stability.
Regarding stem design, we believe that a porous (rather than polished) stem will provide better initial stability and promotes bone incorporation later on, ensuring long-term durability. However, inserting or extracting porous stems is more technically demanding. To exclude the effect of stem design when evaluating the impact of stem diameter and length, we only included patients with the same prosthesis type and stem design.
Regarding RTKR prosthesis fixation, the hybrid approach typically involves using a longer diaphyseal-engaging stem with cement fixation over the epiphyseal and metaphyseal areas, whereas fully cemented fixation often involves using a shorter stem with cementation of the entire prosthesis-bone interface. Fehring et al. reviewed 475 RTKAs and reported more RLLs for hybrid than for fully cemented fixation [8], while Greene et al. reported no aseptic loosening on midterm follow-up of 119 RTKAs with hybrid fixation [14]. Similarly, Edwards et al. reported less radiographic loosening for hybrid-fixation RTKA involving a two-stage exchange arthroplasty protocol for periprosthetic joint infection [7]. Edwards et al. also found that hybrid fixation was not associated with increased infection rate despite using less antibiotic-loaded cement. In their radiostereometric analysis, Heesterbeek et al. found no fixation-related difference in prosthesis micromotion following RTKA [15]. Because there is ongoing controversy regarding which type of fixation provides better stability, we only included RTKA with hybrid fixation, to exclude the effect of such confounding factors when assessing the influence of stem characteristics on stability.
Adequate reconstruction of the bone defect is key for long-term stability of the knee prosthesis. Various methods for reconstruction of more severe bone defects have been reported (e.g., allograft, cone, sleeve), with good clinical outcomes [16,17,18]. A stable initial environment for incorporating the bone graft can be achieved using the extension stem, which can provide prosthesis stability, ensure correct alignment, and spread the loading of the prosthesis-bone junction, thus protecting the bone graft from early failure [19]. Completo et al. confirmed that the stem can spread the loading across the bone graft-cement interface, on both the femoral and tibial side [4, 20]. In the current study, multivariate analysis revealed bone defect severity (AORI type IIa) as a risk factor for loosening at the femoral but not the tibial side. While increased defect severity understandably compromises prosthesis stability, the AORI classification is somewhat arbitrary, and the distinction between type I and type IIa may be subjective (high inter-observer disagreement). Therefore, the effect of bone defect severity on prosthesis stability may not be as substantial in patients with mild defects, which could also explain why we found no correlation between tibial bone defect and tibial loosening. No other reports have indicated that AORI type I and IIa defects would have a different effect on implant stability. A previous study on RTKA treated AORI type I and IIa defects as a single category [6]. To exclude the effect of very high defect severity, we included only patients with type I/IIa defects.
We found that male sex may be a risk factor for loosening of the femoral component after RTKA, which is similar to previous observations in a Norwegian joint registry [2]. It was hypothesized that higher BMI, more intense use of the prosthetic joint, and malalignment may contribute to early loosening. To the best of our knowledge, no study has evaluated the relationship between knee alignment and RTKA failure, although many have assessed the relationship between alignment and primary TKA loosening. Ritter et al. found that, following primary TKA, the risk of failure is lowest (0.6%) for an overall alignment of 3°–7° valgus and highest for more varus or valgus alignment (1.5 and 1.4%, respectively) [21]. Fang et al. concluded that, in primary TKA, the varus knee tends to fail due to medial tibial collapse, while the valgus knee tends to fail due to ligament instability [22]. In our series, the median overall alignment was 5.0° (IQR, 2.1°–6.2°) and 5.8° (IQR 5.2°–7.4°) among patients with and without loosening, respectively. Most patients with loosening had varus malalignment.
Fracture around the stem can occur when using a stem with loo large a diameter. Cipriano et al. found an incidence of 1 and 4.9% for femur and tibia periprosthetic fracture, respectively, among 634 press-fit stems [23]. All but one such fractures were treated conservatively (one received a cable wire), and all healed uneventfully, without implant loosening. Fortunately, there were no fractures in our series, possibly due to the relatively small sample size. We assessed outcomes using Knee Society scores. Patients with loosening had significantly worse functional scores and total scores, while the clinical scores were only slightly worse. Hardeman et al. also reported slightly worse clinical scores and functional scores in patients with RLL score ≥ 4 versus < 4 [24]. However, an RLL score ≥ 4 might not necessarily indicate prosthesis loosening, since inter- and intra-observer variability may be substantial for such a small cutoff. Therefore, in consideration of the modified Knee Society radiographic scoring system, we defined loosening as an RLL score ≥ 9 for the femur and ≥ 10 for the tibia component [8].
Pain around the stem tip is often noted for stemmed prostheses used in total hip arthroplasty or revision knee arthroplasty. Two of our patients reported shin pain around the tip of the tibial stem, and both had CFR > 0.85. The symptoms were mild and resolved with oral analgesics. Stem tip pain might reflect a mismatch of elastic modulus between the stem and cortical bone [25]. In total hip arthroplasty, such pain could be resolved by plating around the stem tip, a technique later adopted in RTKA [25, 26]. The stem material may also play an important role in stem tip pain. Peters et al. reported an incidence of only 2% for stem tip pain after RTKA with fluted titanium stems [27], while Barrack et al. reported an incidence of 18.8% for solid fluted CoCr stems and 8.1% for slotted titanium stems [28]. In the present series, we used the Zimmer LCCK prosthesis with a titanium stem, which might explain the low incidence of stem tip pain. However, when planning to use stems with larger CFR, the possibility of stem tip pain is non-negligible and should be explained to the patient preoperatively.
We further discuss study limitations other than the retrospective design. First, the retrospective study design make it difficult to set up a definitive guideline for stem choice. Rather, we attempted to understand the importance of CFR by these clinical data. We will try to further prove the results through finite element and biomechanical study in the future. Second, as in other studies [6,7,8], we evaluated prosthesis stability on plain X-ray and used the Knee Society radiographic scoring system to avoid detection of non-significant micromotion. However, this approach might underestimate micromotion. Third, evaluation of bone quality (good, fair, or poor) was based on an arbitrary method [7]. Future studies should apply a more objective approach for assessing bone quality. Fourth, we did not assess collateral ligament status and thus could not exclude the effect of collateral ligament competence on stress loading at the prosthesis-bone junction. Nevertheless, it has been demonstrated that collateral ligaments contribute little to the varus-valgus stability of knee joints with LCCK prostheses, as this design limits ligament elongation [29].
