1 Introduction

The execution of a TKA^2,9,24 is an effective way to treat osteoarthritis, a very common joint disease that causes severe pain and debilitation, and the number of patients presenting valgus deformities represents approximately 10% of the total number of TKAs performed.²⁰ The presence of this kind of deformities requires the surgeon to achieve proper alignment, stability and balance to obtain successful clinical outcomes, especially for knees with high coronal deformities; these goals, however, have proven to be challenging to achieve.^7,16 As a result, the use of constrained prostheses is often recommended for valgus knees.¹² Moreover, patients presenting valgus knee deformity have increased risk of failure respect to the non-pathological configuration²¹: for this reason a proper correction of the deformity should be performed during surgery.²² Several studies addressing alignment correction propose surgical techniques as a potential solution for both significant varus^23,28 and valgus^1,5,10,18,20 knee deformities, this latter one known to be more challenging.^17,19

Even though the use of unconstrained mobile bearing for severe knee deformities is rare,⁶ it has been shown to give successful outcomes.^6,26 Sorrels et al.²⁶ investigated survivorship and postoperative alignment of cementless mobile bearing implants on a wide range of coronal deformities and evaluated a ten-year survival of 89.7% for their severely abnormal alignment group. Czekaj et al.⁶ additionally obtained a survival rate at ten years of 98.6% with severe deformities using a low-constraint deep-dish mobile bearing implant design, showing a high reliability of ultra-congruent mobile bearing prosthesis for malaligned knees. On the other hand, other studies are specific to severe valgus deformities such as Tucker et al.²⁷ and Pagoti et al.¹⁹ who both reported satisfactory results using unconstrained mobile inserts.

Dislocation or spinout are however complications related to mobile bearing implants¹³ and associated to preoperative valgus malalignment^8,11;Song et al.²⁵ also warn about tibial component loosening when using mobile bearing prosthesis in significant varus malalignment. On the other hand, unconstrained mobile bearing allows to reduce wear, which represents a common issue in TKA.

The aim of the study is to evaluate whether the surgical technique involving unconstrained prosthesis used on patients with valgus knees is able to achieve satisfactory clinical results and correct alignment as well as good patient satisfaction. This study thus gathers the results of 69 TKAs performed by a single surgeon with cemented mobile bearing implants and preoperative valgus deformities.

2 Materials and methods

Between October 2014 and November 2018, 69 TKAs were performed by a single surgeon on 67 different patients presenting a preoperative valgus knee deformity (see Fig. 1). Two subjects had both knees replaced. All patients gave their written informed consent to have their clinical records later used for this prospective study. Furthermore, the patients were asked to fill a preoperative WOMAC survey¹⁴ and Numeric Rating Scale (NRS) for pain15, as well as a postoperative satisfaction level and a will re-do test.

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Fig. 1 CT-scan image of valgus knee representative of patient cohort.

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The data, collected prospectively by the surgeon, were analyzed retrospectively. The WOMAC questionnaire is presented as a scale from 0 to 96, where 0 means the patient has any drawback to report and 96 means the patient reports severe problems. The Numeric Rating Scale for pain, from 0 to 10, 0 being no pain and 10 being the maximal pain.

The total preoperative valgus angle was established by evaluating the angle between the anatomical axis of the femur and of the tibia, while the postoperative valgus deformity was defined as the angular difference between the mechanical axis of the femur and the tibia, indicating how far the tibia is from the vertical axis.

All the patients received a cemented mobile bearing implant, and the same technique hereby described was used for every surgery. The implant used consists in three components: the femoral component has a constant radius design in the sagittal plane, and the two distal condyles have a constant radius of curvature in the coronal plane to increase the contact surface with the polyethylene of the insert: this characteristic allows the femoral component to be compatible even with very congruent inserts. The lateral sliding (LS) insert is an asymmetrical insert, thus having the bearing surfaces of the medial and lateral compartments characterized by different geometry. The tibial component is a symmetrical CoCrMo alloy tray with a central hole, designed to accommodate the insert pivot. If additional stability is required to the implant, a stem extension can also be hosted (among the patients cohort, only one stem extension was required).

The surgical procedure performed on all patients was previously described by Castellarin3 and Castellarin and Cimino.⁴ Relying on the use of a dedicated surgical tool, called Extramedullary Alignment System (EMAS, Adler Ortho SpA, Cormano, Italy), an innovative surgical technique was developed and initially employed for TKA with inaccessible femoral canal.³ This technique has shown to be reliable also for standard cases, allowing to perform TKAs without violating the femoral canal.⁴

3 Results

3.1

3.1 Preoperative patient assessment

From a descriptive point of view, 69 TKAs were performed on 67 different patients, including 60 women (89.6%) and 7 men (10.4%) with a mean age of 69.7 years. Two female subjects were operated on both legs. A total of 35 left knees (50.7%) and 34 right knees (49.3%) was replaced. Three groups of knees were defined according to the severity of their valgus deformity: mild (>0° and <5°), moderate (≥5° and <10°) and severe (≥10°); similar classifications are also applied in other studies.^16,26 The number of knees in these groups was respectively 19 (27.5%), 22 (31.9%) and 22 (31.9%): the homogeneous patients numerosity among these groups was defined in order to address in a comparable way all the ranges of deformity levels. Pre-op data from 6 knees went missing before being collected for this study, and thus it was not possible to categorize the relative patients; however, they were part of the selected cohort (therefore presenting at least a mild level of deformity) and for this reason their results were considered for the study, nevertheless.

The sizes of the different prosthesis components implanted are presented in Figs. 2–4.

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Fig. 2 Femoral components implanted and sizes.

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Fig. 3 Insert components implanted and sizes.

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Fig. 4 Tibial components implanted and sizes.

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A total of 67 pre-op WOMAC surveys were collected; the mean result of this evaluation was 15.9 points. The Numeric Rating Scale for pain had an average of 2.2 for 68 tests.

4 Discussion

A correct alignment is crucial to obtain successful surgical outcomes,²² and the results showed a reduction from an average pre-operative total valgus angle of 12.5° to an average post-operative angle of 0.6°. The occurrence of revisions was rare (1 out of 69 operations). No case of dislocation or spinouts was reported. The results also showed the overall high satisfaction level of the patients. This suggests that mobile bearing prosthesis can represent a viable alternative to more constrained implants when addressing valgus knee deformities.

Standard surgical instrumentation usually relies on the femoral intramedullary canal as a reference to guide the distal femoral cut and consequently its alignment on the coronal plane. In general, this involves a limitation in the distal femoral valgus resections from 3° to 9°. It is to be noted that the hypoplastic lateral femoral condyle is very commonly related to valgus knee cases, and this deformation requires a proximal displacement (in the order of some millimeters) of the distal femoral cut²²: as a consequence, thus, the joint line is raised compared to the prior configuration. This modification can then lead to patella baja problems, inducing flexion issue.

After the cut, the surgeon then has to perform the soft tissue balancing to have the two cuts (femoral and tibial) parallel in coronal plane in extension; in flexion, the shape of the joint space depends on the rotation in the coronal plane of the femoral component. Standard instrumentation usually relies on the Whiteside and the epicondylar lines to assess this rotation.²⁸ In many cases, a standard 3° external rotation is assigned to the femoral component. As the femoral rotation and the tibial cut are set independently, surgeons often rely on ligament release to achieve a rectangular space in flexion; if the ligaments are damaged or released to achieve knee alignment, however, stability may have to be obtained through the employment of mechanically constrained implants such as PS, CCK or sometimes even hinged prosthesis.

The approach used by the surgeon in this study, on the other hand, consisted in cutting the tibia a few degrees in varus to already correct some degrees on the tibial side.³ The decision correlated to the size of the tibial cut also depended on the valgus deformity contribution of each bone, as the total valgus level is usually due to a combination of femoral deformity (as the forementioned hypoplastic lateral femoral condyle) and tibial bow.

The EMAS device used in the surgeries involved is a dynamic spacer that can help correcting the deformities and achieving a rectangular space in flexion and extension,⁴ allowing to address the above-described issues without performing ligament release: therefore, the use of an unconstrained implant can be taken into consideration as the stability is not flawed by any soft tissue release and thus acceptable outcomes can be achieved with mobile bearing models also in case of severe valgus cases.

It is to be highlighted that the EMAS device allows a correction of valgus deformities only up to a total valgus angle up to 15°, so in more severe cases constrained implants are still the most viable solution. Consequently, PS TKAs are still used by the surgeon for some applications, although the proportion of these implants in his practice is relatively low. Furthermore, other authors were also able to obtain satisfactory outcomes on more severely deformed knee.^6,19,27

Even though in this study the revision rate was available only over a short period of time, it is worthy to mention that also Sorrels et al.²⁶ obtained comparable revision rate using cementless low contact stress (LCS) mobile bearing design; however, they were not able to obtain a systematically corrected alignment, which was instead achieved most of the times in the present study.

Acknowledgement of some limitations of this study has however to be made: first, a relatively low number of cases were analyzed, and this is partly due to the low occurrence of valgus deformities.²⁰ Furthermore, due to the recentness of the operations analyzed, no long-term post-operative follow-up was available; the results over a shorter period were however satisfactory. Finally, the EMAS device used in the followed technique imposed a limitation of a total preoperative valgus angle up to 15°, above which the EMAS device could not ensure complete correction of the deformation of the knee.

5 Conclusion

Under the right conditions, the described surgical technique (not requiring any soft tissue release or femoral canal accessibility) allowed to achieve correct pot-op alignment and patient satisfaction using mobile bearing implants on knee with valgus deformities. Patient follow-up will be performed to further evaluate long-term outcome.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Informed consent

Informed consent was obtained from all patients.

Declaration of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.