Bone Cut Accuracy in Total Knee Arthroplasty: Do Conventional Cutting Guides Stay True to the Planned Coronal Orientation of the Components?

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The investigations presented in this paper support the existence of a discrepancy between the planned angulation and the final radiological measure on the coronal plane of the femur and tibia using conventional cutting guides. The observed difference between both the pre and post measurements is small enough to suggest that it probably does not affect clinical outcomes. However, it is important to consider this uncontrollable error to optimize ligament balance and postoperative alignment.

Abstract

Background: Total knee arthroplasty (TKA) has become the gold standard for the definitive treatment of knee osteoarthritis. One crucial aspect in the implantation of a TKA is the precise orientation of the femoral and tibial components. The main purpose of the present study is to assess, in patients undergoing total knee replacement, whether the difference between the planned angulation in the distal femoral cut and the angulation obtained in the postoperative radiological control is low enough to consider the conventional bone-cutting guides reliable. Methods: A retrospective study was designed with a consecutive series of patients who had undergone primary total knee arthroplasty using conventional instrumentation over one year. The authors analysed the main variable (bone cuts) while considering different variables (age, gender, surgeon, prosthesis, laterality, constraint, body mass index and alignment) to identify different patient patterns that justify the results in the main variable. Descriptive variables were analysed using the Mann–Whitney U and Kruskal–Wallis tests. Additionally, the correlation between continuous variables was explored in accordance with the Spearman correlation. Results: A total of 340 patients with a mean age of 75 ± 9.16 years were finally included in the present study. The mean absolute error of the main variable for the femoral coronal bone cut was 1.89° (SD 1.53). For the tibial coronal bone cut, it was 1.31° (SD 2.54). These values correspond to what remains after subtracting the radiological angulation obtained in the postoperative period from the planned intraoperative angulation of the distal femoral cut. No associations were observed between the main variable (the angulation of the proximal tibial cut and distal femoral cut) and the rest of variables for either the femur or tibia. Conclusion: A discrepancy between the planned angulation and the final radiological measure on the coronal plane of the femur and tibia using conventional cutting guides has been demonstrated. The degree of deviation is low enough that it probably does not affect clinical outcomes. Therefore, the use of conventional cutting guides will continue to be an appropriate tool to perform bone cuts in knee replacement surgery.

1. Introduction

In recent years, orthopaedic surgeons worldwide have given significant importance to lower extremity alignment when implanting a total knee arthroplasty. However, in recent studies, little attention has been paid to the precision with which implants have been positioned using conventional cutting guides. The substantial number of patients arriving at medical institutions with this medical condition has pushed the orthopaedic surgeon towards finding ways to solve this problem in the most adequate manner. Total knee arthroplasty (TKA) is the gold standard for the definitive treatment of late-stage osteoarthritis [1,2].

One crucial aspect in the implantation of a TKA is the precise orientation of the femoral and tibial components [3]. The incorrect placement of the prosthetic components or poor coverage of bone surfaces [4,5] and the subsequent incorrect postoperative alignment can lead to a poor functional result and aseptic loosening of the implant due to inadequate biomechanical functioning and the subsequent increased wear of the polyethylene [6,7,8]. In addition, several authors suggested that an alignment within 3° of the varus or valgus provides the best long-term TKA survivorship [9,10].

For this reason, some authors have proposed various studies to assess the effectiveness and accuracy of bone cuts when TKAs are performed. Navigation and patient-specific instrumentation (PSI) were once considered newer and more precise alternatives to conventional cutting guides [8,11].

However, the percentage of cases that fall outside the accepted values are between 5% and 21% when we use navigation as an additional guidance method to perform bone cutting [8,12,13]. In the same way, some authors also think that PSI does not translate into greater precision and that it does not make for better clinical or radiological results [14].

Over the years, some surgeons have considered modifying the distal femoral bone cut to improve implant stability [15]. However, it is especially important that this modification be translated into the final radiological result to assess whether it is crucial to achieving said objective or, on the contrary, if this stabilization is produced by other compensation mechanisms, meaning that modifying the cutting angle is not necessary. The authors of this study consider it important to assess the difference found between the intraoperative planning and the final radiological result and to be aware of the need to perform these calculations. If the difference between the planned and actual is very great, it probably does not make sense to justify this coronal alignment philosophy.

The main purpose of the present study is to assess, in patients undergoing total knee replacement, whether the difference between the planned angulation in the distal femoral cut and proximal tibial cut and the angulation obtained in the postoperative radiological control is low enough to consider the conventional bone-cutting guides reliable. The secondary objective of the present study is to identify if there is a particular pattern in our patients that makes it to observe this difference.

2. Materials and Methods

A retrospective study was designed with a consecutive series of patients who had undergone a primary total knee arthroplasty using conventional instrumentation over one year.

The inclusion criteria were patients between 50 and 85 years old with primary knee osteoarthritis (all patients were classified as grades III (34%) and IV (66%) on the Kellgren-Lawrence scale [16]). The exclusion criteria were active or suspected sepsis, a tumour around the knee, a previous TKA, the presence of hardware that could leave artefacts, any extra-articular deformity that required osteotomy around the knee in conjunction with TKA and other rheumatic disease. Patients who were diagnosed with osteoporosis and patients who needed a tibial or femoral coronal recut during surgery were excluded because both conditioning elements might suppose an added factor for imprecision.

Four senior surgeons from the same institution performed all the procedures following the same pre, intra- and post-operative protocols. Four different implants were used: the Triathlon Knee System^® (Stryker, Mahwah, NJ, USA), the U2 Knee System TKA^® (United Orthopedic Corporation, New Taipei City, Taiwan), the NexGen LPS Knee System^® (Zimmer, Irvine, CA, USA) and the Genutech Knee System^® (Surgival, Paterna, VC, Spain). The surgeons decided to use postero-stabilized (PS) system or cruciate-retaining (CR) system, depending on the posterior cruciate ligament integrity.

The surgical technique was standardized for all surgeons. For the procedures, a tourniquet and intradural anaesthesia were used with all patients. Femoral and sciatic blocks were used for pain control during the postoperative period. Prior to all procedures, antibiotic prophylaxis and tranexamic acid were administered to the patients. A standard midline anterior skin incision and a medial parapatellar deep approach were performed. In terms of the bone cutting, an intramedullary guide was used for the distal femoral cut, and an extramedullary guide was used for the proximal tibial cut in all surgeries. The femoral intramedullary guide was always placed beyond the femoral isthmus, and the tibial extramedullary guide was always placed with the same reference at the mid-malleolar point. The surgeons aimed at 90° for the tibial cut and a variable degree for the coronal femoral cut in all procedures, depending on the alignment or ligament balance encountered in each patient. The distal femoral cut was planned based on long leg X-rays while considering the angle between the mechanical and anatomic femoral axis; however, the surgeon always had the option to change the femoral distal cut angle up to three degrees depending on the ligament balance (gap-balancing technique). The femoral and tibial coronal cuts were made with a 1.27 mm thick sawblade. All implants, femoral and tibial, were cemented only on the surface of the component in all patients and were well impacted until no cement exuded from the sides of the prosthesis. Patellar replacement was performed on all the patients.

The operation used to calculate the main variable was always calculated by subtracting the real angle measured in the X-ray from the desired angle. All cutting angles were recorded in a surgical report. Two independent observers measured the anatomical lateral distal femoral angle (aLDFA) and mechanical medial proximal tibial angle (mMPTA) in the six-month follow-up using long leg X-rays. Patient positioning, in terms of rotation and extension for the X-ray projection, followed strict guidelines (patient standing, with knees fully extended and patellae facing forward. The patella needed to always be centred and aligned with the femoral condyles. Otherwise, the technician modified the rotation of the limb until the desired position was achieved.) [17]. The Picture Archiving and Communication System (PACS) was used by both observers to perform measure collection, all to one decimal point. The final analysis was performed with the average measure of both observers.

Quantitative data from the difference between the desired angle and the measured angle were expressed as means and standard deviations. Descriptive variables relating to the patient (e.g., body mass index, age (in three groups: <65, 65–79 and >79 years) or gender) and other intrinsic aspects of the surgical procedure (e.g., the model of the prosthesis, laterality, constraint (PS and CR) or surgeon) were also described. As the sample distribution of differences was highly skewed to the right and the normality test (Shapiro–Wilk) clearly rejected the normal distribution hypothesis, non-parametric options were applied. Comparisons between groups were performed with the Mann–Whitney U and Kruskal–Wallis tests. Using Spearman’s correlation, the correlation between continuous variables was explored in accordance with the non-parametric test. An interclass correlation test was conducted to evaluate the correlation between observers. A Bland and Altman plot was performed to evaluate the difference between both observers. All statistical analyses were performed in STATA 15.1 (STATA Corp LP, College Station, TX, USA). Statistical significance was set with a p-value of <0.05.

This study was performed in line with the principles of the Declaration of Helsinki. The local ethical committee of our institution approved the study (2016/6604/I). Informed consent was obtained from all participants included in the study.

3. Results

A total of 463 patients were initially identified. We excluded 123 participants due to the exclusion criteria: 25 because of a double bone cut in surgery and 98 due to osteoporotic pathologies. Thus, 340 patients were finally selected for the analysis. The mean age was 75 ± 9.16 years. The majority of the participants were females 262 (77%). There were 167 left knees (49.1%) and 173 right knees (50.9%). In terms of constraint, 322 patients were operated on with the postero-stabilized (PS) system and 18 with the cruciate-retaining (CR) system. No mechanical complications were observed in the present study.

The mean absolute error of the main variable (the desired angle less the measured angle) for the distal femoral and proximal tibial bone cuts, considering that the observers collected the difference between the planned surgical angle and the real angle observed in X-rays (planned minus real), tended towards varus deviation for both the femoral (1.89°, SD 1.53) and tibial (1.31°, SD 2.54) cuts.

From at the outliers, it can be observed that 14.7% (50) of the patients had a discrepancy of more than 3° in the difference between the aimed and real femoral coronal angle, and 18.2% (62) had more than a 3° difference in the tibial frontal angle.

No associations were observed between the main variable and the variables of age (divided into three age groups) for either the femur or tibia, gender, laterality, constraint, implant and surgeon (

Table 1. Mean deviation in bone cuts considering the variables of age (divided into three age groups), gender, laterality, constraint, implant and surgeon.

	Femoral Cut					Tibial Cut
	Group	n	Mean	SD	p-Value	Group	n	Mean	SD	p-Value
Age	<65	34	1.96	1.55	0.76	<65	34	0.60	2.91	0.18
	65–79	194	1.83	1.53		65–79	194	1.38	2.15
	>79	112	1.98	1.62		>79	112	1.34	2.11
Laterality	Left	167	1.92	1.42	0.36	Left	167	1.10	2.18	0.09
	Right	173	1.86	1.68		Right	173	1.51	2.28
Gender	Male	76	1.77	1.40	0.61	Male	76	1.27	2.70	0.84
	Female	264	1.93	1.60		Female	264	1.32	2.10
Constraint	PS	322	1.90	1.55	0.65	PS	322	1.31	2.25	0.81
	CR	18	1.76	1.64		CR	18	1.44	2.09
Prosthesis	NexGen	60	2	1.42	0.71	NexGen	60	1.33	2.24	0.48
	Triathlon	134	1.82	1.57		Triathlon	134	1.30	2.15
	U2	42	1.80	1.54		U2	42	1.78	2.20
	Genutech	104	1.97	1.62		Genutech	104	1.13	2.37
Surgeon	1	85	1.77	1.31	0.45	1	85	1.27	2.09	0.08
	2	122	1.89	1.40		2	122	1.76	2.04
	3	56	1.86	1.59		3	56	1.08	1.99
	4	77	1.63	1.33		4	77	1.03	2.36

(SD: standard deviation; n: sample size; PS: postero-stabilized; CR: cruciate-retaining).

).

We did not find any association or correlation between the body mass index (BMI) or preoperative deformity relative to the femoral or tibial cut (

Table 2. Mean deviation in bone cuts considering the BMI and preoperative alignment variables.

	Femoral Cut					Tibial Cut
	Group	n	Mean	SD	p-Value	Group	n	Mean	SD	p-Value
BMI	<25	29	1.58	1.45	0.21	<25	29	0.96	2.11	0.12
	>25–<30	114	1.75	1.56		>25–<30	114	1.33	1.93
	>30–<35	111	1.92	1.49		>30–<35	111	1.37	2.39
	>35	86	2.15	1.65		>35	86	1.93	2.38
Preoperative alignment	<160°	7	1.14	0.89	0.22	<160°	7	2	1.33	0.18
	160–165°	16	2.56	2.63		160–165°	16	0.86	2.55
	165–170°	57	2	1.30		165–170°	57	1.05	2.20
	170–175°	151	1.74	1.56		170–175°	151	1.26	2.17
	175–180°	109	2	1.48		175–180°	109	1.48	2.33

(SD: standard deviation; n: sample size; BMI: body mass index).

). The interclass correlation coefficient between both observers was 0.599; CI 95% (0.503–0.667).

4. Discussion

The most interesting finding in this study, due to the surgeon’s awareness of a cutting error, is that there was a discrepancy (1.89° in the femur and 1.32° in the tibia) between what the surgeon planned intra-operatively and the final real measure when conventional bone-cutting guides were used. The deviation in the femur and in the tibia tended to a varus cut. This result leads us to the realization that there is some uncontrollable error in terms of alignment and stability that must be considered when surgeons a plan knee replacement. Nevertheless, the amount of this discrepancy has been described in several articles as a non-affecting factor for clinical outcomes [18,19]. Additionally, some other authors observed that the different angulation in the femoral distal bone cut does not influence its precision [20].

This mismatch in terms of accuracy lets the surgeon know that the approach must be applied with certain considerations in mind. When presented with a valgus knee in a safe zone in terms of alignment, it may be safer to stay between the limits because the tendency to error leans toward the varus. However, stability may be more difficult to achieve because the lateral compartment tends to be tighter. The varus knee must be analysed in the opposite sense (Figure 1).

The difference between the planned measures and the result after surgery can be analysed by taking several parameters into account. Leeuwen et al. [11] suggested that possible explanations for this discrepancy might be found in inaccuracies in the identification of landmarks, the fixation of the cutting blocks or the rigidity of the sawblade. Kim et al. [21] concluded that femoral shaft bowing may have an influence on the final coronal femoral alignment due to the angulation of the femoral intramedullary guide. One question worth considering is whether cement thickness can be a cause, depending on the final compression the surgeon performs after positioning the implants. Inadequate ligament balancing may lead to an irregular prosthesis–surface contact caused by more compression on one compartment than the other [22]. In addition, the use of different conventional cutting-guide systems may be a reason for the discrepancies between them. However, in the present study, the four instruments demonstrated similarities in precision with no significant differences.

Moreover, we can justify the lack of coincidence due to the measurements being derived from X-rays. In a study performed by Li et al. [23], a Bland–Altman plot demonstrated sufficient agreement to justify using the CT scan and X-rays interchangeably. Bowman et al. [24] evaluated the reliability of studies with measurements based on long leg radiographs, and they concluded that the post-operative evaluation could be performed with these radiological studies. However, intra- or inter-observer reliability appears to be better when the measurements are carried out on a CT scan. The high variance between assessments might be explained by two important factors: the accuracy of the landmarks and patient positioning at the time of the X-ray. Boonen et al. [25] showed differences between the pre-planning and postoperative measurements of 1.8° and 1.7° on the coronal plane of the femur and the tibia, respectively. Those assessments were made with a 3D CT scan in consideration of a lack of significant differences. Leeuwen et al. [11], measuring postoperatively with a 2D CT scan, found smaller differences of 1.2° in the coronal femur and 0.4° in the coronal tibia when using PSI technology. Considering our results and comparing them with other studies, it is easy to think that two of the most important factors involved in obtaining the most accuracy in measuring are probably the observers’ training at the time of evaluation and having a strict imaging protocol in place in terms of patient positioning (axial and sagittal planes), following the indications published by Cooke et al. [26] and Jamali et al. [27].

In the analysis of the patient profiles in which bone cut accuracy was worse, the present study did not identify a clear, describable pattern for either obesity or preoperative deformity based on the results seen in the literature. Over the years, the BMI has been one of the most concerning factors when the surgeon attempts to position the implant. Yoo et al. [28] stated that there were no differences in tibial component alignment between stratified BMI groups. Shetty et al. [29] observed no significant differences between obese and non-obese patients in terms of femoral and tibial component alignment. Deformity prior to surgery is another important factor to analyse when considering the accuracy of bone cutting. León-Muñoz et al. [30] observed that accuracy tends to diminish in bone cuts in patients with more than 10 degrees of varus in the hip-knee-ankle (HKA) angle. In that study, patients operated on with the CT-based PSI system saw a decrease in accuracy of over 15° of varus in the HKA. In our study, that association was also found.

If one looks at the different technological systems aimed at helping surgeons achieve bone cut accuracy, the literature also reports discrepancies. Seon et al. [31] showed interesting results in terms of accuracy using PSI guides. They saw a discrepancy of 0.5° in the distal femur and 0.1° in the coronal proximal tibia. However, that was with a limited number of patients. Then again, other studies [32] have shown a high proportion of outliers when using this PSI technology, or the same degree of accuracy when comparing PSI with conventional cutting guides [33]. Navigation systems may be a reliable tool to obtain precise results in bone cuts. Ulivi et al. [34] observed that 95.5% and 90.1% of the patients were within the range of 3° varus/valgus in the tibia and femur, respectively. A recent Goh et al. study [35] demonstrated that even with accelerometer- and navigation-improved alignment, there were no differences in clinical outcomes, quality-of-life and satisfaction when compared to conventional instruments. Regarding robotics, there are several studies defending their greater accuracy in bone cutting [36,37,38]. However, further studies with longer follow-ups are needed to confirm the benefits of this new technology. In a recent meta-analysis, Bouché et al. [39], concluded that better and more consistent studies are needed to support the use of those technologies when comparing the precision of the PSI, the accelerometer, navigation, robotics and conventional cutting guides. The proportion of outliers (+/−3°) in coronal femur alignment was lower for the accelerometer group when compared with the PSI (1% vs. 11%) or conventional (1% vs. 14%) groups. Regarding frontal tibial outliers, navigation demonstrated the lowest proportion when compared with standard cutting guides (5% vs. 10%) or with the PSI group (5% vs. 10%). In the present study, the outliers (+/−3° referencing the aimed angulation) were 14.7% in the frontal femur (50) and 18.2% (62) in the coronal tibia.

Remaining mindful of these results and considering the discrepancy observed in the present study, the accuracy of conventional cutting guides should be considered acceptable with no need for additional technology.

There are three main limitations of the present study. The first is the use of full-leg standing radiographs to measure the angles, acknowledging that it is probably less accurate than a CT scan. However, both observers were trained together to identify the landmarks in the same manner, and a strict patient positioning protocol was adhered to. The protocol was overseen by the radiologist in charge of performing specific imaging studies. The radiologist was involved from the moment our general database was created. The second limitation is the lack of measurements on the sagittal and axial planes. Nevertheless, the most objective planned measures are on the coronal plane. It is difficult to define the planned sagittal (with the extramedullary guide) and rotational angulation in the tibia and the planned rotational and sagittal position in the femur due to the fact that inexact references were chosen using conventional instrumentation. Furthermore, a major additional limitation is we could not assess the intra-observer correlation because the observers automatically calculated the mean value of their two measurements. In the cases in which the measurements were more than 0.5, they assessed a third measurement.

5. Conclusions

In conclusion, the existence of a discrepancy between the planned angulation and the final radiological measure on the coronal plane of the femur and tibia using conventional cutting guides has been demonstrated. The degree of deviation is low enough that it probably does not affect clinical outcomes. However, it is important to take this uncontrollable error into account to optimize ligament balance and postoperative alignment. The use of conventional cutting guides will continue to be an appropriate tool to perform bone cuts in knee replacement surgery.