Introduction

Total knee replacement (TKR) is claimed to be the most efficient method of treating end-stage osteoarthritis of the knee joint. TKR aims to relieve patients of pain, restore range of motion and improve the quality of patients’ lives. It is a widely performed procedure, but many patients’ results are dissatisfying. Studies have shown that approximately one-fifth of patients who have undergone TKR remain dissatisfied [1]. Many factors influence radiological and clinical outcomes and patient contentment after TKR. They can be divided into several groups. These include factors associated with the operation technique and patient and implant-related factors. One of these is the shape of the prosthesis components. Prostheses design is still being modified and improved through the cooperation of orthopaedic surgeons and engineers. These efforts aim to maximise patients’ satisfaction with the procedure [2]. This is possible, among other things, thanks to access to increasingly progressive production techniques and advanced materials. Some of the most advanced solutions aimed at matching the anatomy of native patients as closely as possible are anatomical knee systems.

Materials and methods

The literature was reviewed, and articles related to the subject were researched. Therefore, publications including keywords such as total knee replacement, anatomic implants, and morphometric implants were searched in the PubMed, Cochrane and Google Scholar databases. The research focused on English-language papers, articles, available abstracts, and studies performed on people. The inclusion of articles was determined based on titles, then abstracts, and eventually entire articles. Articles were eliminated if found to reflect any signs of unreliability or irrelevance to the topic. Duplicates or outdated information were removed. Thirty-six articles were analysed, out of which 20 were selected. Research and error risk evaluation was performed by one author. All data was selected and verified individually. Analysis and synthesis of studies were prepared separately.

Results

The shape and size of femoral and tibial components vary between individuals, especially between women and men. Hence, the perfect adjustment of standard implants characterised by limited diversity can be very challenging (Fig. 1 and 2) [3].
Anatomical prostheses offer asymmetrical tibial baseplates and a broader range of available sizes compared to predecessor models. The possibility of using a larger variety of sizes decreases the risk of both tibial and femoral overhang compared to the non-morphometric one [4]. Misalignment of implant and bone shape, such as tibial and femoral overhang, may cause chronic knee pain, decreased range of motion (mainly flexion) and worsened functional outcome [4-6]. The implemented anatomical tibial component increases tibial coverage compared to symmetrical designs and allows for proper rotational alignment, the reconstruction of which can be limited in conventional designs (Fig. 3) [7].
Anatomical design not only enables better alignment of the prosthesis elements with the native femoral and tibial bone structure but also allows the surgeon to resect less bone tissue. Saving as much tissue as possible may be relevant for possible revision surgery in the future [8].
One of the goals of total knee replacement is to restore the reduced range of motion. This parameter is, therefore, improved regardless of the type of implant used. However, when comparing two cruciate-retaining systems: a morphogenic and a non-morphogenic knee system, it is clear that the use of anatomic implants offers the chance of a greater improvement in range of motion than the implantation of a non-morphogenic system [9].
As the study shows, posterior condylar offset increases in anatomical and standard TKR groups of patients who underwent TKR, but the increase in the anatomical-implant group was higher than in the non-anatomical implant group. There seems to be a positive correlation between posterior condylar offset and restoration of the range of motion, especially flexion. On the other hand, another research [10] revealed that more does not always mean better, and significantly increased posterior condylar offset may not influence flexion. According to the study, patients whose posterior condylar offset increased in the range of 0-2 mm benefit the most. Higher offset is also related to lesser bone loss [4]. In the context of the lesser bone loss mentioned a while ago, it is worth noting that one of the anatomical design knee implant systems has the smallest femoral box of all known knee implants and allows for the most sparing bone resection in the intercondylar groove for the Insall box [11].
Furthermore, a broader range of component sizes and shapes may increase the compatibility level between femoral component parts and result in a lack of need to derogate from joint space balancing [6].
Standard implant designs have focused on the best tibial coverage at the expense and risk of implant malrotation [12]. The implemented anatomical prosthetic tibial component enhances tibial coverage and enables more accurate rotation alignment, the reconstruction of which may be limited in conventional designs. Asymmetric designs are more resistant to non-ideal rotation restoration [7]. To enable proper rotation alignment on non-anatomic systems, the coverage must be compromised. However, while obtaining proper rotation seems to be the more critical factor, tibial coverage is not negligible. According to the study, it affects, e.g., tibial bone resorption, patients’ prognosis, and the surgery result [13].
Morphometric total knee arthroplasty is a relatively new concept in orthopaedic practice, and its survivorship and functional outcome must be assessed in the more distant future. However, as research shows [14], the evaluation of anatomic implants at a minimum 5 years follow-up revealed excellent radiological functional outcomes assessed by several functional scores.
Patellofemoral joint reconstruction is another element of the successful operation. Trochlear anatomy is crucial in proper patellar tracking. Patellar malalignment and related abnormal distribution of forces and kinematics may lead to anterior knee pain. Prosthetic trochlear geometry is an essential factor in patellofemoral joint restoration after total knee arthroplasty. Morphogenic implants decrease the risk of trochlear dysplasia [15].
The use of anatomical implants permits reconstruction as close as possible to the native anatomy and function with high precision, taking into account the comparison of pre-and post-operative radiographs after total knee replacement [16].
According to the study, the introduction of a new total knee replacement system was related to the excess intraoperative blood loss and increased longevity of the surgery immediately after its implementation, which raises the risk of perioperative complications. However, this problem occurs at the initial stage of surgeons’ contact with the system. The mentioned problems cease to be relevant after a year of surgeons’ experience with the system [17].
Following the arguments above, it can be concluded that anatomical systems may result in better clinical and radiological outcomes [18]. Nonetheless, further development and refinement of total knee prostheses designs is still needed, which can result in an ideal adjustment of the prosthesis to the native bone shape, leading to improvements in both clinical and radiological postoperative results and the level of patients’ satisfaction after total knee replacement surgeries [19].
The next step may be the development of anatomic and gender-specific knee implant systems. Given that women constitute the majority of patients undergoing total knee arthroplasty surgery and that women are not „little men”, their specific anatomy and needs should be considered when developing subsequent models. As studies show, some women benefit from the use of gender-specific knee implants [20, 21].

Conclusion

Anatomical implants enable better alignment of the prosthesis elements and the native femoral and tibial bone structure than non-anatomical ones. Morphometric models make it possible to obtain a greater range of motion and, consequently, improved functional outcomes compared to non-morphometric ones. It also enables less bone resection and better radiological outcomes.
However, no endoprosthesis would perfectly replicate the anatomy and biomechanics of a healthy knee joint, so models of knee endoprostheses must be constantly improved and modified [19, 22].

References

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