The prevalence of posterolateral tibial baseplate overhang: An anatomical CT study

Abstract

Introduction

Total Knee Arthroplasty (TKA) is used in patients with osteoarthritis who have failed conservative management to reduce pain, improve functional outcomes and ultimately quality of life. However, less than optimal patient satisfaction has led to continued improvements in design and technology of TKA. One factor that can limit patient satisfaction is postero-lateral overhang of the tibial baseplate. The purpose of our study is to utilize pre-operative CT scans to assess the prevalence of posterolateral tibial overhang with the use of a symmetric tibial baseplate component in a popular knee system with robotic assistance.

Methods

Ninety-eight (98) consecutive patients who underwent robotic-assisted total knee arthroplasty (TKA) were included in this study. Using both the most medial aspect of the tibial tubercle and the medial ⅓ of the tibial tubercle as reference points, we quantified the extent of posterolateral overhang when determining the rotation of the tibial component.

Results

Using the most medial aspect of the tibial tubercle as a reference point for rotation of the tibial baseplate, 63% of the instances of reviewed CT scans (369/588) had posterolateral overhang. Furthermore, 81% (406/588) had posterolateral overhang when using the medial ⅓ aspect of the tibial tubercle as the reference for rotation of the tibial baseplate. The average posterolateral tibial baseplate overhang was 1.5 mm (range 0–8 mm) when using the most medial aspect of the tibial tubercle and 2.4 mm (range 0–8 mm) when using the medial ⅓ tibial tubercle as the centering point for the tibial baseplate.

Discussion

Tibial baseplate overhang could lead to potential pain from irritation of soft tissues. To our knowledge, this is the first study that was able to valencquantify the amount of tibial baseplate overhang using pre-operative CT scans. Rotational alignment of the tibial baseplate needs to be balanced to ensure minimal lateral overhang while achieving sufficient external rotation of the tibial component. An asymmetric tibial component may provide a compromise in certain situations.

Level of evidence

Diagnostic level IV case series.

1. Introduction

Knee replacement surgery has been used for almost 50 years and its usage is growing. Goals of knee replacement surgery are to reduce pain, improve functional outcomes and ultimately quality of life. However, even after decades of design and technique improvements, patient dissatisfaction rates are still being reported to be as high as 20%.1, 2, 3, 4 In select groups, those dissatisfaction rates can be over 50%, such as patients younger than 55 with mild arthritis.⁵ Patient satisfaction can depend on many factors, including residual pain, stiffness and swelling, and individual patient's pre-operative expectations for their outcome.² Common reasons for revisions are implant loosening, infection, pain, and instability.⁴ Persistent post-operative pain can be attributed to intra-articular versus extra-articular factors. Intra-articular factors are associated with aseptic loosening, polyethylene wear, instability, patellar mal-tracking, tendon ruptures, stiffness or oversized implants.^6,7 Oversized implants can generate significant pain after knee replacement due to soft tissue impingement and overstuffing of the knee joint.⁸ It is well documented that oversized femoral components can cause clinically important persistent postoperative knee pain, with overhang happening more often in women and shorter patients.^9,10 Research has shown contradicting conclusions regarding functional outcomes in patients with oversized tibial components. Some suggest that tibial tray overhang or under-sizing do not significantly affect outcomes compared to anatomically sized controls,¹¹ while others state that medio-lateral and antero-posterior oversizing is frequent and have lower functional outcomes.⁸ Many anatomical studies have investigated gender, ethnicity, morphology-type, individual and even intra-personal variations in knee parameters and their effect on component placement.^10,12, 13, 14, 15, 16, 17, 18 Due to the anatomic variety described, most total knee systems have off-the-shelf standard femoral and tibial component sizes. Some total knee systems do not offer asymmetric sizes of tibial trays leading the arthroplasty surgeon to either accept slight component overhang or downsize the component to avoid overhang in certain patients whose morphology requires it. Undersized components carry a risk of failure, as it has been shown that components that lack cortical support are at risk for subsidence.¹⁹ Furthermore, with better radiological outcomes compared to conventional TKA, Robotic assisted TKA systems are increasing in implementation and popularity with higher accuracy of component placement in both uni-compartmental and total knee replacements.^20,21 However, increased accuracy may not clinically translate to improved outcome. The purpose of our study is to assess the prevalence and extent of posterolateral tibial overhang with the use of symmetric tibial baseplate component in the Stryker MAKO PS Knee system (Kalamazoo, MI) using the tibial tubercle as a reference of component rotation.

2. Methods

Ninety-eight (98) consecutive patients who underwent robotic-assisted total knee arthroplasty (TKA) were included in the study (Table 1). The surgeries were all performed by the same surgeon (HFD) at an urban hospital from 2016 to 2018. After receiving approval form the institutional review board, the patient data was accessed on the Mako system. Individually, six observers used the Mako software to evaluate positioning of the tibial component that was implanted based on the pre-operative computed tomography (CT) scan of the patient.

Table 1.

Demographics data of study population.

N = 98
Age	Mean = 59	Range 36-80
Gender	Female	69
	Male	29
Race	African-American	45
	Caucasian	23
	Middle Eastern	6
	Hispanic	3
	Unknown/Other	21

The observers used the software to assess the position of the component on the tibial plateau. The depth of the tibial cut and component size were not altered from the executed operative plan. The rotation of the component in the axial plane can change the extent of overhang. Each data collector measured the tibial overhang using two different methods of determining the axial rotation of the tibial component. The first method utilizeda line between the posterior cruciate ligament (PCL) insertion and the most medial aspect of the tibial tubercle (Fig. 1). The second method utilizedutili\ a line between the PCL insertion and the medial ⅓ of the tibial tubercle (Fig. 2).

Fig. 1.

Demonstrates the axial rotation of the tibial component using a line between the PCL insertion (blue dot A) and the most medial aspect of the tibial tubercle (marked at the tip of the upper green arrow in B).

Fig. 2.

Demonstrated the axial rotation of the tibial component using a line between the PCL insertion (blue dot in image A) and the medial 1/3rd of the tibial tubercle (green arrow in image B).

After setting the axial rotation of the component, we then focused on the translational alignment in the axial and coronal planes. The component was placed in such a way that it would provide coverage and ensure sufficient supporting bone stock of the tibial plateau without overhang, exactly as the surgeon would place the component in the operating room. Once the component was properly aligned, we measured the distance from the most posterolateral aspect of the bone cut to the tibial component. If the component did not overhang on the lateral plateau, then the measurement was recorded as 0 mm. Measurements were made in millimeters and were rounded to the nearest whole number.

For each patient, the lateral overhang was measured using two methods. First, with the axial rotation of the component set by a line between the PCL insertion and the most medial aspect of the tibial tubercle (demonstrated in Fig. 1). A second measurement was then recorded, using the axial rotation by a line from the PCL insertion to the medial ⅓ of the tubercle (demonstrated in Fig. 2). An example of posterolateral overhang is shown in Fig. 3.

Fig. 3.

The component was adjusted to where it would be placed intra-operatively. The optimal position included no anterior overhang and equal coverage of the medial and lateral plateaus (image A). The postero-lateral overhang was then measured to the nearest millimeter using the measuring crosshair (image B).

The six researchers who made these measurements made them at different times and were blinded to the other researchers’ measurements until after the overhang values were obtained. The values for each of the 98 patients were averaged in order to estimate the lateral overhang of the tibial component after a robotic-assisted total knee arthroplasty using both the medial aspect of the tibial tubercle as well as the medial ⅓ of the tibial tubercle.

2.1. Statistical analysis

The primary outcome of interest in this study was understand the prevalence and average overhang distance from Mako surgery. Inter-rater agreement from different observers was evaluated using reliability analysis (Cronbach's Alpha test). For all analyses, P value smaller than 0.05 was considered statistically significant. All analyses were performed using SPSS software (Version 25, IBM, Chicago, IL).

3. Results

98 consecutive patients with pre-operative CT scan prior to total knee arthroplasty (TKA) by a single surgeon (HFD) between the dates of January 2016 through November 2018. Six observers independently obtained measurements for the 98 consecutive patients (Table 1). The calculations are based off 588 measurements (98 CT scans independently measured by six observers for a total of 588 measurements) for each reference point. When using the most medial aspect of the tibial tubercle as a reference point for rotation of the tibial baseplate, 63% of CT scans reviewed (369/588) had posterolateral overhang. Furthermore, 81% (406/588) had posterolateral overhang when using the medial ⅓ aspect of the tibial tubercle as the reference for rotation of the tibial baseplate. The average posterolateral tibial baseplate overhang was 1.5 mm (range 0–8 mm) when using the most medial aspect of the tibial tubercle and 2.4 mm (range 0–8 mm) when using the medial ⅓ tibial tubercle as the centering point for the tibial baseplate (Table 2). Table 2 also demonstrates statistical analysis amidst the six researchers the six researchthe who participated in this study. Cronbachs' alpha reliability analysis demonstrated that there was agreement among the six raters for the distance of overhang measured for the most medial aspect of the tibial tubercle (Alpha = 0.731), and the medial ⅓ of the tubercle (Alpha = 0.727).

Table 2.

Inter-rater reliability of overhang measurement.

Observer	Measurement (mm) (Mean ± SEM)						Cronbach Alpha
	1	2	3	4	5	6
Most Medial TT	1.8 ± 0.2	1.4 ± 0.2	1.9 ± 0.1	0.6 ± 0.1	1.5 ± 0.1	1.8 ± 0.2	0.731
Medial 1/3 TT	2.7 ± 0.2	2.2 ± 0.2	3.0 ± 0.1	2.3 ± 0.2	2.3 ± 0.2	1.8 ± 0.2	0.727

4. Discussion

In total knee arthroplasty (TKA), the surgeon is tasked with obtaining the best fit tibial baseplate for each patient. In some instances, the surgeon must use his/her judgement as to whether an undersized or oversized component would be the most appropriate. Overhang could lead to potential pain from irritation of soft tissues, while an undersized component could potentially lead to subsidence and subsequently failure of the implant. Finite element analysis models and clinical outcomes research in the past have been used to investigate the modes of failure of the tibial components in TKA, component malposition being an important contributing factor.^22,23 Varus and valgus malpositioning can lead to ligamentous failure and increased contact stresses on the polyethylene inserts,²³ however, little information is available regarding axial alignment of the tibial component. Based on previous FEA studies, it was suggested that placing the tibial component more lateral is favorable to decrease medial cortex stresses and, as a result, decrease bone resorption in a situation of an undersized tibial component.²⁴ It has also been published in previous studies that rotational alignment may play a more significant role in TKA kinematics than medial-lateral component placement.²⁵ External rotation of the tibial component was found to reduce retropatellar pressures and correlate with longer implant survivorship.^26,27 Our study aimed at using pre-operative computer tomography (CT) scans for patients undergoing TKA with the Makoplasty system^Ⓡ and to evaluate how prevalent tibial baseplate overhang based off the method we described. Based on previous research, external rotation of the tibial component by aligning it to the medial third of the tibial tuberosity can help optimize knee kinematics and improve implant longevity. Our study found that when placing the tibial baseplate directly centered over the most medial aspect of the tibial tubercle led to an average of 1.5 mm of overhang and when the tibial baseplate was centered over the medial third of the tibial tubercle we found an average of 2.4 mm of overhang. It is interesting to note that in our study population of 98 patients there was a significant increase in the overhang when using the medial third tibial tubercle to center the tibial baseplate as compared to the most medial aspect of the tibial tubercle. Previous studies have shown that posterolateral overhang leads to differences in patient-reported outcomes including a higher incidence of posterolateral knee pain as well as lower overall outcome scores.²⁸ As robotic-assisted arthroplasty becomes more prevalent in its use, it is important to evaluate the technical aspects of the procedure and how they relate to biomechanical and clinical outcomes. To our knowledge, this study is the first to report of whether or not placement of the symmetrical tibial component in the Makoplasty software leads to posterolateral overhang. This is an anatomical descriptive study which is the first step in describing and quantifying the extent of posterolateral overhang while contemplating the placement of a symmetrical tibial component in the medial-lateral rotational planes. During intra-operative planning, appropriate placement of the tibial component is determined by overlaying the component over the patient's CT scan. The baseplate is centered in the coronal, sagittal, and axial planes in order to achieve the best possible fit on the medial and lateral plateaus without overhang. In an ideal world, the baseplate would fit perfectly on the cortical bone along all edges of the plateau. This study demonstrates that an ideal fit of currently-available components may lead to posterolateral overhang in some patients.

This data can be used in multiple ways, by both surgeons as well as orthopaedic implant companies. Using advanced imaging, we have quantified the amount posterolateral overhang. Surgeons can take this into account during intra-operative planning and try to minimize overhang during axial positioning, while keeping in mind that external rotation of the component (using medial third of the tibial tubercle as reference instead of the most medial aspect of the tibial tubercle) can increase the posterolateral overhang. Orthopaedic implant companies can use this data, along with other studies that show lower patient outcome scores with posterolateral overhang, and may consider use of an asymmetric tibial component or a customized, patient-specific component for robotic-assisted procedures in certain patients.

This study has some limitations. Firstly, the patient data that was evaluated was from a single surgeon's practice. However, since each blinded author made their own measurements and adjusted the tibial baseplate with their own intra-operative planning decisions, the fact that the patient data came from a single practice did not affect the outcome of the measurements. Another limitation is that there is inherent subjectivity in the individual baseplate placement and measurements as is true with any study that relies on an individual's assessment. Our authors were blinded to each other's measurements. Averages of the measurements were used in order to help overcome this subjectivity. Additionally, statistical analysis demonstrated inter-observer reliability. The final limitation of the study is the nature of an anatomical descriptive study which does not include clinical correlations or patient outcomes in the analysis. This can be addressed with further investigation into clinical outcomes in the next study.

In conclusion, this study is unique in that it is the first to evaluate posterolateral overhang of a symmetrical tibial component in a robotic-assisted total knee arthroplasty using pre-operative CT scans. Furthermore, we were able to quantify the amount of overhang based on two commonly used reference points for tibial baseplate placement i.e. medial aspect and the medial third of the tibial tubercle. It remains to be answered if the amount of overhang we measured is clinically significant to the patient, further studies are needed to assess how much overhang is clinically significant. Currently, we can account as best as we can for this overhang during intra-operative planning and try to minimize the amount. It is important for the arthroplasty surgeon to consider which reference point to use for tibial baseplate placement to balance the overall fit of the component and its rotational alignment.