Clinical outcomes and radiograph assessment
Patients were followed up at 3, 6, and 12 months post-operatively and yearly thereafter. Clinical outcomes were evaluated in terms of knee Hospital for Special Surgery (HSS) score, visual analog scale (VAS) for pain, knee ROM, and presence of any complications. Any complications such as deep venous thrombosis, pulmonary embolus, deep infection, arthritis of the lateral compartment, or loosening were also recorded. Additionally, the final assessment was recorded for analysis.
Weight-bearing anteroposterior, lateral, and full-length radiographs were obtained at our institution both pre-operatively and post-operatively. Care was taken to ensure that each patient stood with their patellae facing forward, to minimize rotational variation among anteroposterior radiographs. Post-operative implant position and alignment were assessed according to the guideline proposed by the Oxford group. The medial tibial bone cut and joint-line change were measured on weight-bearing anteroposterior radiographs using the following method: on the pre-operative radiograph, both the anatomical axis of the tibia (line a) and a line perpendicular to the anatomical axis from the lowest point of the medial tibia (line b) were drawn. The distance from line b to the peak point of the tibial vertices was measured (distance α). On the post-operative radiograph, the perpendicular line (line c) to the anatomical axis (line a) from the bottom of tibia implant was drawn. Similarly, the perpendicular line (line d) to the anatomical axis (line a) from the top surface of the bearing was also drawn. The distance β and γ from the same peak point of the tibial vertices to the lines c and d were measured. The difference between distance α and distance β was defined as the medial tibial bone cut amount. The difference between distance α and distance γ was defined as the joint line change [Figure 3].
All data were analyzed using SPSS 17.0 (SPSS Inc., Chicago, IL, USA). Data are reported as the mean with the standard deviation. The Chi-square test and Student's t test were used to determine whether statistically significant differences existed between the groups. A P-value <0.05 was considered statistically significant.
All 68 patients were included at the final follow-up in October 2018. Twenty knees (29.4%) were from male patients and 48 knees (70.6%) were female patients. The mean follow-up was 38.2 ± 6.3 months and was at least 2 years for all patients (range, 27–51 months). At baseline, the mean age at the time of the operation was 68.4 ± 7.5 years (range, 54–81 years). The mean body mass index was 25.2 ± 2.7 kg/m2. In total, 36 UKA procedures were performed on the right knee while 32 were performed on the left knee. The mean pre-operative ROM was 121.4° ± 10.2°, which was improved to 127.7° ± 6.6° at the final follow-up (t = –4.55, P < 0.001). The mean HSS knee score increased from 58.7 ± 7.7 to 92.4 ± 5.1 at the time of the final follow-up (t = –27.72, P < 0.001). The mean VAS score was reduced from 6.8 ± 0.9 to 2.0 ± 0.8 (t = 34.81, P < 0.001). The mean pre-operative HKA on weight-bearing radiographs was 174.0° ± 3.3°, which was corrected to 177.3° ± 2.1° on the post-operative radiographs (t = –9.37, P < 0.001) [Table 1].
There were no significant differences in age, sex distribution, body mass index, pre-operative ROM, HSS score, VAS score, and HKA between the two groups. For the modified group and conventional group, the mean follow-up time was 38.2 ± 6.2 months and 38.2 ± 6.6 months, respectively (t = –0.01, P
= 0.992), and the mean post-operative ROM was 128.1° ± 7.0°, 127.4° ± 6.2°, respectively (t = –0.44, P
= 0.659). There were no significant differences in post-operative HSS score (91.3 ± 4.8 vs. 93.4 ± 5.2, t = 1.69, P
= 0.095) and VAS score (2.1 ± 0.8 vs. 1.8 ± 0.8, t = –1.65, P
= 0.104) between the two groups. Furthermore, no significant differences were found in the pre-operative and post-operative HKA between the two groups (pre-operative, 173.5° ± 3.0° vs. 174.5° ± 3.5°, t = 1.26, P
= 0.210; post-operative, 177.4° ± 1.7° vs. 177.1° ± 2.4°, t = –0.62, P
= 0.535) [Table 1].
Significant differences were noted between groups with respect to the proximal tibial bone cut, distal femoral bone mill, and joint line change in the coronal planes. The amount of proximal tibial bone cut was significantly less in the modified group than in the conventional group (4.7 ± 1.1 mm vs. 6.7 ± 1.3 mm, t = 6.45, P < 0.001). Correspondingly, there was a significant difference in distal femoral bone mill (4.9 ± 0.8 mm vs. 3.4 ± 0.9 mm, t = –7.83, P < 0.001). The joint line of medial compartment was slightly elevated by 2.1 ± 1.0 mm in the modified group compared with –0.5 ± 1.7 mm in the conventional group (t = –7.46, P < 0.001) [Table 1]. The tibial implant size in the modified group was larger than that in the conventional group (χ2 = 4.95, P = 0.035), whereas the bearing size and femoral size were similar in the two groups (χ2 = 1.34, P = 0.569; χ2 = 0.76, P = 1.000, respectively) [Table 2].
No patients in either group underwent conversion to TKA or experienced serious adverse events related to the operation in both groups, such as death, periprosthetic joint infection, pulmonary embolism, or a cardio-cerebral vascular incident. One bearing dislocation occurred 2 months after surgery in the conventional group because of hyperflexion trauma. This bearing was replaced by a thicker one. One patient in the modified group reported persistent unexplained knee pain.
According to the guidelines proposed in the surgical manual of the Oxford group, the post-operative radiographic assessments showed that the accuracies of the post-operative position and alignment of implanted prostheses were comparable between the two groups. Clinically acceptable overall implant alignment and position were achieved in 97.1% of patients in each group. In the modified group, one femoral component tilted in the coronal plane with a post-operative radiographic angle >10°. In the conventional group, one case was under-corrected in the coronal alignment along with a posterior tibial slope >12° in the sagittal plane. Neither of these patients with implant failure had clinical symptoms at the last follow-up.
The most important finding of the present study is that the modified technique for tibial bone sparing is reliable and has many advantages. The modified technique can reduce the amount of tibial bone cut, which is useful in reducing the tibial bone defect in revision. Furthermore, it can help to achieve a larger cement surface on the tibial side, which would benefit patients of smaller size. Finally and most importantly, the modified technique is comparable with the conventional technique in terms of clinical outcomes and radiographic assessments.
The major advantage of the tibial bone sparing technique in UKA is that it preserves the tibial bone mass by about 2 mm, which is useful in reducing the tibial bone defect during revision. According to the national registry report, UKA had a relatively high revision rate compared with TKA. Furthermore, Chawla et al reviewed 124 studies and reported that the annual revision rate of UKA was 1.07%, which was 2.18-fold of TKA. Among the many other failure modes, aseptic loosening was the main failure mode in UKA.[15,16] In most cases of UKA failure, bone defects occur on the tibial side and require revision devices such as metal augments, bone grafts, and stems when converting to TKA. The femoral bone defects are usually small and contained, and they may only require autologous morselized bone grafts in most cases. However, the tibial side bone loss in a failed UKA is a major problem, especially when a metal-backed implant has been used. Possible causes of bone loss include loosening with component subsidence, osteolysis from wear, removal of the components and cement, the metal-backed design, and the bone cut in the primary operation; among these, the most prominent causes were the metal-backed design and the bone cut. Therefore, a more primary tibial bone cut might result in a large bone defect if UKA fails in future. As described in the pre-operative X-ray image of the primary TKA brochure, the tibial bone horizontal cut line usually lies 8 to 10 mm below the lateral compartment surface in varus knee. Namely, the amount of the proximal tibial cut is 2 mm below the surface of the medial side in anteromedial knee osteoarthritis. In conversion to TKA, if the tibial bone horizontal cut line is drawn below the tibial component, it does not influence the TKA procedure. However, this was not found to be the case in our practice. In the clinical practice of primary Oxford UKA, the thickness of bone removed from the tibia must be enough to accommodate a tibial tray (3 mm) and a bearing that is usually 4 mm thick. Therefore, the amount of tibial side gap after bone cut in Oxford UKA is about 7 mm, not including the tibial groove. For this reason, conversion from Oxford UKA to TKA is very different from primary TKA and is more technically difficult to perform successfully.[18,19] In the case shown in Figure 4, the tibial bone loss in a failed UKA was usually 5 mm more than that in a primary TKA, which is primarily due to the native tibial bone cut (usually 5 mm more than TKA on the medial side). If a surgeon cuts more bone in the primary operation and the tibial bone defect is larger than 10 mm in the revision, then the revision might need a metal block/wedge, a bone graft, or a tibial stem.[11,12] Hence, it is very important to preserve the tibial bone mass. As shown in the present study, the modified cutting technique can preserve the tibial bone mass by about 2 mm, which decreases the tibial bone defect by about 2 mm in the revision. Reducing the tibial bone cut by 2 mm might result in a revision without the need for a stem or bone graft.
Another advantage of reducing the tibial bone cut is increased medial support and decreased pain during medial stress. The bone strength of the tibia significantly decreases as the distance from the subchondral resection surface increases. A significant tibial bone cut leads to a weaker bone bed for the tibial component. This modified technique advances the tibial bone cut closer to the joint space, thus basing it on an optimal tibial bone surface. Additionally, it can help to achieve a larger tibial plate on the tibial side, which is crucial in dispersing stress. Although no consensus has been reached regarding the tibial bone cut thickness in UKA, it is better to minimize the amount of tibial osteotomy to prevent tibial fracture and decrease the risk of implant subsidence or loosening. Excessive tibial resection may increase the forces on the tibia surface and cause pain. In a biomechanical study, Simpson et al demonstrated that after UKA, the mean strain on the proximal tibial cortex increased by 6%, 13%, and 18% when tibial resection levels of 2, 4, and 6 mm were modeled, respectively. Furthermore, Small et al demonstrated similar results: 4 mm increased distal bone cut increased tibial strain variance by 35%. Therefore, surgeons must be particularly careful to avoid an overly-excessive tibial bone cut.
A third advantage of the tibial bone sparing technique in UKA is that a smaller tibial cut allows for a larger tibial size. Because the proximal tibial bone is an inverted triangle in the coronal plane, a larger amount of bone cut will lead to a smaller tibial surface. Hence, a smaller tibial cut could allow for a larger tibial surface. As a result, the tibial implant size could potentially be larger. The advantage is obvious for Asian patients who have a relatively small body size. Statistically speaking, in the Asian population, smaller values have been reported for the medial tibia, where the only available size of tibia prosthesis is sometimes too large. In the present study, size A was used on the tibial side in most cases. In Chinese patients with a small body size, the smallest tibial size (size AA) has been reported to overhang. This overhanging tibial component will irritate the synovial membrane and eventually cause pain. Additionally, large tibial coverage has been shown to disperse stress. Small tibial coverage may induce a collapse or loosening of the tibial plateau.[25,26] Hence, a smaller tibial cut will result in a larger tibial size and greater coverage, which will largely benefit patients with a small body size by reducing stress and pain.
Most importantly, the modified technique is comparable with the conventional technique in terms of the clinical outcomes and radiographic assessments. Although the joint line was slightly elevated in the modified technique group of the present study, there were no significant differences in the HSS score, VAS score, ROM, or radiographic assessments between the two groups. Clinically acceptable overall implant alignment and position were achieved in 97.1% of patients in each group. No conversion to TKA or any serious operation-related adverse events had occurred after a minimum 2-year follow-up. Notably, these acceptable outcomes depend on a slight joint line change. A significant joint line elevation may influence the knee kinematics, alignment, ROM, and overall results.[27,28] Although the acceptable amount of joint line elevation is controversial, Takayama et al found that a medial tibial joint line elevation of >5 mm restrained the improvement in the knee extension angle in UKA.
This study has several limitations. First, this study was limited to early post-operative results. Further studies are required, including those that provide subsequent clinical results and revision reports. Second, the sample size of this experiment was relatively small. A well-designed randomized controlled study with a larger sample size might provide more significant results. Third, the sample in this study was limited to the Chinese population. Whether this method is also applicable in patients who generally have a larger body size, such as the European and American populations, remains unclear. For more significant and generalizable results, a random sample taken from multiple populations is; therefore, required.
Despite these limitations, the clinical relevance of the present study is that the modified tibial bone sparing technique is reliable and easily performed. The post-operative radiographic and clinical results are equally as good as those in the current conventional technique.
The authors thank the patients included in the study for their cooperation. The authors thank Lei Wang for his support to draw the diagram and prepare the video for this study. The authors also like to thank Mr. Omar M. Fakhr (Department of Biological Science, The University of Tulsa, Tulsa, OK, USA) for the English improvement.
This study was funded by the National Natural Science Foundation of China (No. 81703896), Beijing Municipal Science and Technology Commission (No. Z171100001017209), and the Capital Health Research and Development of Special (No. 2016-2-4062).
Conflicts of Interest
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Unicompartmental knee arthroplasty; Tibial bone cut; Surgical technique; Radiologic; Clinical outcome
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