Arthroplasty Surgeons Differ in Their Intraoperative Soft Tissue Assessments: A Study in Human Cadavers to Quantify Surgical Decision-making in TKA

Abstract

Background

In TKA, soft tissue balancing is assessed through manual intraoperative trialing. This assessment is a physical examination via manually applied forces at the ankle, generating varus and valgus moments at the knee while the surgeon visualizes the lateral and medial gaps at the joint line. Based on this examination, important surgical decisions are made that influence knee stability, such as choosing the polyethylene insert thickness. Yet, the applied forces and the assessed gaps in this examination represent a qualitative art that relies on each surgeon’s intuition, experience, and training. Therefore, the extent of variation among surgeons in conducting this exam, in terms of applied loads and assessed gaps, is unknown. Moreover, whether variability in the applied loads yields different surgical decisions, such as choice of insert thickness, is also unclear. Thus, surgeons and developers have no basis for deciding to what extent the applied loads need to be standardized and controlled during a knee balance exam in TKA.

Questions/purposes

(1) Do the applied moments in soft tissue assessment differ among surgeons? (2) Do the assessed gaps in soft tissue assessment differ among surgeons? (3) Is the choice of insert thickness associated with the applied moments?

Methods

Seven independent human cadaveric nonarthritic lower extremities from pelvis to toe were acquired (including five females and two males with a mean age of 73 ± 7 years and a mean BMI of 25.8 ± 3.8 kg/m²). Posterior cruciate ligament substituting (posterior stabilized) TKA was performed only on the right knees. Five fellowship-trained knee surgeons (with 24, 15, 15, 7, and 6 years of clinical experience) and one chief orthopaedic resident independently examined soft tissue balance in each knee in extension (0° of flexion), midflexion (30° of flexion), and flexion (90° of flexion) and selected a polyethylene insert based on their assessment. Pliable force sensors were wrapped around the leg to measure the loads applied by each surgeon. A three-dimensional (3D) motion capture system was used to measure knee kinematics and a dynamic analysis software was used to estimate the medial and lateral gaps. We assessed (1) whether surgeons applied different moments by comparing the mean applied moment by surgeons in extension, midflexion, and flexion using repeated measures (RM)-ANOVA (p < 0.05 was assumed significantly different); (2) whether surgeons assessed different gaps by comparing the mean medial and lateral gaps in extension, midflexion, and flexion using RM-ANOVA (p < 0.05 was assumed significantly different); and (3) whether the applied moments in extension, midflexion, and flexion were associated with the insert thickness choice using a generalized estimating equation (p < 0.05 was assumed a significant association).

Results

The applied moments differed among surgeons, with the largest mean differences occurring in varus in midflexion (16.5 Nm; p = 0.02) and flexion (7.9 Nm; p < 0.001). The measured gaps differed among surgeons at all flexion angles, with the largest mean difference occurring in flexion (1.1 ± 0.4 mm; p < 0.001). In all knees except one, the choice of insert thickness varied by l mm among surgeons. The choice of insert thickness was weakly associated with the applied moments in varus (β = -0.06 ± 0.02 [95% confidence interval -0.11 to -0.01]; p = 0.03) and valgus (β = -0.09 ± 0.03 [95% CI -0.18 to -0.01]; p= 0.03) in extension and in varus in flexion (β = -0.11 ± 0.04 [95% CI -0.22 to 0.00]; p = 0.04). To put our findings in context, the greatest regression coefficient (β = -0.11) indicates that for every 9-Nm increase in the applied varus moment (that is, 22 N of force applied to the foot assuming a shank length of 0.4 m), the choice of insert thickness decreased by 1 mm.

Conclusion

In TKA soft tissue assessment in a human cadaver model, five surgeons and one chief resident applied different moments in midflexion and flexion and targeted different gaps in extension, midflexion, and flexion. A weak association between the applied moments in extension and flexion and the insert choice was observed. Our results indicate that in the manual assessment of soft tissue, changes in the applied moments of 9 and 11 Nm (22 to 27 N on the surgeons’ hands) in flexion and extension, respectively, yielded at least a 1-mm change in choice of insert thickness. The choice of insert thickness may be more sensitive to the applied moments in in vivo surgery because the surgeon is allowed a greater array of choices beyond insert thickness.

Clinical Relevance

Among five arthroplasty surgeons with different levels of experience and a chief resident, subjective soft tissue assessment yielded 1 to 2 mm of variation in their choice of insert thickness. Therefore, developers of tools to standardize soft tissue assessment in TKA should consider controlling the force applied by the surgeon to better control for variations in insert selection.

Introduction

In TKA, soft tissue balance is typically assessed through intraoperative trialing. In essence, this is a physical examination maneuver that involves manually applying forces in the medial and lateral directions to the ankle, generating varus and valgus moments at the knee while visualizing the lateral and medial gaps at the joint line [6, 14, 22, 39]. Based on this examination, surgical decisions are made, including polyethylene insert thickness and whether ligament releases or additional bony resections should be performed [6, 21, 39]. These decisions are important contributors to postoperative knee mechanics and joint stability. An improper soft tissue assessment can compromise knee stability, which could contribute to patient dissatisfaction and implant loosening [2, 25, 34].

Despite the pivotal role of intraoperative trialing, the magnitude of the applied forces is not standardized, and the resulting gaps are based on a subjective assessment [6, 17]. Thus, this exam remains a qualitative art that relies on the intuition, experience, and training of each surgeon. A variety of instruments have been developed to quantify the assessed gaps and contact forces intraoperatively via navigation systems and intraarticular force sensors to inform surgical decisions [3, 10, 12, 37]. However, these tools do not account for the magnitude of forces applied by the surgeon during gap assessment; thus, interpretation of these outputs may be confounded by variations in applied load. Tensor devices that distract the knee in extension and flexion have the advantage of controlling the distraction force [23, 31]. However, these devices do not replicate how surgeons examine the knee intraoperatively [23, 35].

Controlling the applied loads during intraoperative trialing could be essential for consistent soft tissue assessment among surgeons. Still, the extent of variation among surgeons in conducting this exam, in terms of applied loads and assessed gaps, is unknown. Thus, whether variability in this manual exam influences common surgical decisions including the choice of insert thickness, remains unknown. Altogether, whether it is necessary to standardize not only the resulting gaps but also the applied forces remain unclear. Standardizing techniques of intraoperative trialing could improve the consistency of soft tissue assessment. Tools with standardized inputs (that is, applied forces) could help identify intraoperative targets for knee stability towards addressing patient dissatisfaction due to knee instability [9, 38].

Therefore, we posed the following research questions: (1) Do the applied moments in soft tissue assessment differ among surgeons? (2) Do the assessed gaps in soft tissue assessment differ among surgeons? (3) Is the choice of insert thickness associated with the applied moments?

Materials and Methods

Overview

This was an experimental study performed in human cadavers. In it, five arthroplasty surgeons (PKS, DJM, MBC, ADP, GHW) and a chief resident (CAK) performed intraoperative trialing examinations on seven human cadaveric legs (age > 60 years) implanted with posterior cruciate ligament substituting (posterior stabilized) TKA implants. Exclusion criteria were arthritis (assessed via radiographs before procurement), deformity, contracture, or injury. The protocol consisted of six steps.

TKA Procedure

Seven fresh-frozen (at -20° C) lower extremities from the pelvis to the toe were used to simulate the in vivo intraoperative conditions during TKA (consisting of independent cadavers from five females and two males. The cadavers had a mean age of 73 ± 7 years and a mean BMI of 25.8 ± 3.8 kg/m²) (Table 1). The TKA procedure was performed on the right knee of each cadaver. Cadavers were thawed to room temperature before surgery. Mechanically aligned TKA was performed by an arthroplasty surgeon (PKS, who performs approximately 250 TKA/year after 6 years in clinical practice) using a standard measured resection technique with manual instrumentation [1]. Specifically, the femoral component was externally rotated 3° relative to the posterior condylar axis, and the tibial tray was internally rotated such that its center was aligned with the medial one-third of the tibial tubercle [18]. The femoral component and the tibial tray were sized for each cadaver (Table 1).

Table 1.

The demographics of the seven human cadavers and the implant size used in the TKA procedure

Human cadavers	Cadaver 1	Cadaver 2	Cadaver 3	Cadaver 4	Cadaver 5	Cadaver 6	Cadaver 7
Sex	Male	Female	Female	Female	Male	Female	Female
Race	White	White	White	White	White	Black	White
Age in years	76	68	68	83	73	64	79
Weight in lbs	148	119	180	139	158	163	140
BMI in kg/m2	21.2	23.2	31.9	24.6	25.5	29.8	24.8
Femoral component size (Persona™ system, Zimmer)	9	6	8	5	11	6	6
Tibial tray size (Persona system)	E	C	E	C	F	C	D

Femoral component sizes of the Persona posteriorly stabilized system ranges from 3 (smallest size) to 11 (largest size); Tibial tray sizes of the Persona posteriorly stabilized system ranges from C (smallest size) to H (largest size).

A posterior cruciate ligament substituting (posterior stabilized), fixed-bearing, Zimmer Persona™ system (Zimmer Biomet) was utilized. This system has insert thicknesses in 1-mm increments over the ranges of 10 to 14 mm and 16 to 20 mm. The tibial tray, the polyethylene insert, and the femoral component were side specific. After placing the trial components, the operating surgeon conducted a standard soft tissue assessment and target the polyethylene insert that achieved medial and lateral gaps in extension less than or equal to 1 mm, medial gap in flexion of no more than 1 mm, and lateral gap in flexion at most 2 mm.

Installation of Reflective Markers for Motion Capture

After TKA, two clusters of reflective spheres were fixed to the femur and tibia anteriorly at a distance 10 cm above and below the joint line, respectively, via 4-mm-diameter bone screws (Fig. 1). These were the closest locations to the joint line that enabled the surgeons to examine the knee without disturbing the reflective markers. Each cluster consisted of three, 9-mm diameter reflective spheres. In addition, four metal pins, each with a diameter of 6 mm, were drilled into the pelvis, leaving 10 cm protruding to fix the cadaver to the testing table.

Fig. 1.

A-E The experimental setup included a human cadaveric torso and limbs resting on a dissection table, with TKA performed on the right knee. The setup included (A) a fixation method to fix the pelvis to the testing table, including a base plate clamped to the table and an external-fixation system securing the pelvis to the plate; (B) four motion capture cameras; (C) four clusters of reflective spheres anchored into the tibia and the femur using bone screws; (D) four force transducers wrapped around the distal tibia and foot, two medially and two laterally; and (E) a video camera. ©2021 Virginia Ferrante-Iqbal.

CT Scanning of Cadaver Legs to Define the Anatomic Knee Coordinate System

All seven cadaver legs underwent CT scanning (Inveon, Siemens Inc) preoperatively to identify anatomical landmarks on the femur and the tibia to develop an anatomical knee coordinate system following the definition of Grood and Suntay [7]. Scanning parameters were 0.6-mm slice thickness and 0.5 x 0.5 mm² in-plane pixel dimensions. All the cadaver legs underwent a second CT scan after all TKA components, sphere clusters, and pelvis pins were installed to determine the location of the reflective marker with respect to the anatomical landmarks identified in the preoperative CT scan. This information enabled registration of the motion capture data to the anatomical coordinate system for quantification of the medial and lateral gaps as detailed in the last step of this method.

Performance of Intraoperative Trialing Examinations

The pins protruding from the pelvis were locked to a mounting plate using an external fixation system (DePuy Synthes) that was rigidly secured to the testing table using C-clamps (Fig. 1A). Four motion capture cameras were placed around the testing table and positioned to ensure that the reflective spheres remained visible within the calibrated volume while examining the lower leg (Motion Analysis Corp) (Fig. 1B-C). Four pliable 4-mm thick force transducers were wrapped around the foot and distal tibia (Loadpad SAF, Novel Electronics) (Fig. 1D). Five fellowship-trained knee surgeons (with 24 (GHW), 15 (DJM), 15 (ADP), 7 (MBC), and 6 (PKS) years of clinical practice in TKA) and one chief orthopaedic resident (CAK) independently evaluated soft tissue balance for each knee. Each surgeon could trial different inserts. Surgeons used their right hands to examine the right knee of each cadaver. The applied forces could differ between the dominant and nondominant hands. However, all surgeons were right-handed and thus used their dominant hand. Surgeons could place their hands as they preferred to replicate their actual clinical technique. We documented the hand positions of the five surgeons and the chief resident during the knee balance exam (Supplementary Digital Content 1; http://links.lww.com/CORR/A768). Subsequently, surgeons chose the insert thickness they thought balanced each knee. We recorded each surgeon’s choice (Table 2). All surgeons were blinded to the decisions of the other surgeons. The chosen insert was installed, and the motion capture cameras and force transducers were activated. The surgeons were then instructed to conduct the intraoperative trialing examination by applying varus and valgus knee moments at approximately 0° of flexion (extension), at approximately 30° of flexion (midflexion), and at approximately 90° of flexion (flexion) (Fig. 2). We relied on each surgeon’s perception to attain the targeted flexion angles, as is done intraoperatively. The surgeon who performed the first test returned after all other surgeons had completed testing and confirmed their initial insert choice to verify that the soft tissue of each knee was not compromised from repeated testing. All six surgeons successfully assessed six cadaver knees. For cadaver knee 6, the medial collateral ligament ruptured while the fourth surgeon was testing it. The examining surgeon heard an audible tear and subsequently observed a notable increase in valgus laxity. Thus, data were collected from the first three surgeons only for this knee.

Table 2.

The thickness of the polyethylene insert (mm) chosen by five surgeons and a chief resident across seven cadavers

Surgeon	Cadaver 1	Cadaver 2	Cadaver 3	Cadaver 4	Cadaver 5	Cadaver 6	Cadaver 7
Surgeon 1	12	13	10	12	11		12
Surgeon 2	11	14	10	10	10	13	12
Surgeon 3	12	14	11	11	11		13
Surgeon 4	11	13	10	11	11	13	12
Surgeon 5	12	14	10	12	10	14	13
Surgeon 6 (chief resident)	12	13	10	12	10		12
Largest variation in insert thickness choice in mm	1	1	1	2	1	1	1

Surgeons 1, 3, and 6 did not choose insert thickness for cadaver 6 (the empty cells) because the soft tissue of this knee was compromised during the soft tissue exam of surgeon 3.

Fig. 2.

A-C This image shows a surgeon examining knee balance on a right human cadaveric leg by applying varus and valgus moments at (A) extension (approximately 0° of flexion), (B) midflexion (approximately 30° of flexion), and (C) flexion (approximately 90° of flexion). The surgeon visualized the lateral and medial gaps at the knee. Four force transducers were wrapped around the distal tibia and the foot to measure the forces applied by the surgeon in the medial and lateral directions to generate varus and valgus moments at the knee. The transducers transmitted the force data wirelessly via Bluetooth to a software application enabling continuous data collection and display. ©2021 Virginia Ferrante-Iqbal.

Measurement of Varus and Valgus Moments

We used force transducers capable of measuring the total force applied to the sensor area [26]. Two transducers were placed around both the medial and lateral aspects of the foot and distal tibia. This sensor configuration isolated the applied forces in the medial and lateral directions from the force applied by the surgeon to grip the cadaver leg and covered all possible areas of hand placement during the examination (Fig. 1). The maximum medially and laterally directed forces were scaled by the location where each surgeon grasped the leg distally to quantify the varus and valgus knee moments (Supplementary Digital Content 2; http://links.lww.com/CORR/A769).

Quantification of Lateral and Medial Gaps

To quantify the lateral and medial gaps, we reconstructed the 3D geometries of the femur and tibia using preoperative and postoperative CT images (Mimics Research 22.0; Materialise) [7]. The 3D geometries of the reflective spheres were segmented and reconstructed from the postoperative CT image and then registered to their respective motion capture data using dynamic analysis software (ADAMS v19, MSC Software Corp) [8]. The lateral and medial gaps were then calculated (Supplementary Digital Content 2; http://links.lww.com/CORR/A769).

Ethical Approval

Ethical approval for this study was obtained from the Hospital for Special Surgery (number 2016-0071-CR1).

Data Analysis

The applied varus and valgus moments (measured with a resolution of 0.1 Nm) and the measured medial and lateral gaps (the calibration accuracy of the motion capture was < 0.1 mm) at the three flexion angles were compared among surgeons. After confirming the data were normally distributed via the Shapiro Wilk test (p > 0.11), we used repeated measures (RM)-ANOVA to determine whether the magnitude of moments and gaps differed among surgeons. A sample size of seven was required to detect differences in the applied moments of 0.6 Nm, with 80% power and an alpha value of 0.05. A moment of 0.6 Nm corresponds to an average knee rotation of 0.25° in varus or valgus [32], which was deemed sufficient to differentiate between the participating surgeons. We used generalized estimating equations to test whether the applied moments and insert thickness choice were related [11]. This method accounts for the repeated measure of six surgeons on the same cadaver and yields a slope (β), confidence interval, and p values of the relationship. Specifically, the slope, β, quantifies the strength of the relationship between applied moment and insert thickness, whereas the CI is a measure of the uncertainty of this relationship. A p value of less than 0.05 was designated as a statistically significant relationship.

Results

Applied Moments Differed Among Surgeons

The applied varus and valgus moments in midflexion and flexion differed between surgeons (Fig. 3). The largest mean differences occurred in midflexion in varus (16.5 Nm; p = 0.02) and in valgus (9.7 Nm; p = 0.03), and in flexion in varus (7.9 Nm; p < 0.001) and valgus (6.7 Nm; p = 0.03), whereas no difference was found in extension in varus (mean difference 6.5 Nm; p = 0.07) or in valgus (smallest mean difference of 2.7 Nm; p = 0.60) (Table 3). Across all cadavers, the largest moment was applied in valgus in extension (13.3 ± 1.1 Nm), and the smallest moment was applied in valgus in flexion (8.9 ± 2.3 Nm) (Table 3).

Fig. 3.

A-C The maximum applied varus and valgus moments during the knee balance examinations among surgeons (Surgeon 1 (S1) to Surgeon 6, (S6) arranged by seniority; Surgeon 1 was the most senior surgeon and Surgeon 6 was the chief resident) at (A) extension, (B) midflexion, and (C) flexion.

Table 3.

Comparing the applied moments in knee soft tissue assessment between five surgeons and a chief resident using RM-ANOVA

	Applied moment in extension (Nm)		Applied moment in midflexion (Nm)		Applied moment in flexion (Nm)
Surgeon	Varus	Valgus	Varus	Valgus	Varus	Valgus
Group (S1) n = 6	10.7 ± 5.9	13.5 ± 4.6	5.7 ± 2.1	6.7 ± 3.0	5.2 ± 3.3	5.3 ± 3.8
Group (S2) n = 7	10.1 ± 5.5	14.3 ± 5.1	9.0 ± 5.7	12.5 ± 4.3	6.6 ± 2.4	9.9 ± 5.1
Group (S3) n = 6	9.8 ± 3.1	12.3 ± 4.5	13.1 ± 3.9	11.6 ± 7.0	12.5 ± 4.8	10.2 ± 3.8
Group (S4) n = 7	16.3 ± 7.6	12.6 ± 3.9	13.3 ± 4.1	8.1 ± 3.7	12.6 ± 4.5	7.9 ± 3.2
Group (S5) n = 7	15.4 ± 4.8	15.0 ± 4.7	22.2 ± 6.9	16.4 ± 7.1	13.1 ± 4.7	12.0 ± 3.7
Group (S6) n = 6	10.8 ± 1.6	12.3 ± 3.4	8.7 ± 2.5	11.0 ± 5.2	10.8 ± 5.5	8.2 ± 2.5
Mean ± SD	12.2 ± 2.9	13.3 ± 1.1	12.0 ± 5.8	11.0 ± 3.4	10.1 ± 3.4	8.9 ± 2.3
95% CI	(11.3-13.1)	(13.0-13.6)	(10.2-13.8)	(9.9-12.1)	(9.0-11.2)	(8.2-9.6)
p value	0.07	0.60	0.02	0.03	< 0.001	0.03

S1 to S6: Surgeon 1 to Surgeon 6 (S6 is the chief resident); mean = mean applied moments.

Measured Gaps Differed Among Surgeons

The lateral and medial gaps differed between surgeons at all flexion angles, except the lateral gap in midflexion (Fig. 4). The largest difference in the mean gap was the lateral gap in flexion, reaching 1.1 mm (Table 4). The smallest difference in the mean gap was the medial gap in extension reaching 0.6 mm (Table 4). We recorded the raw data of the medial and lateral gaps (Supplementary Digital Content 3; http://links.lww.com/CORR/A770).

Fig. 4.

A-C The measured medial and lateral gaps during the knee balance examinations among surgeons (Surgeon 1 to Surgeon 6, arranged by seniority; S1 was the most senior surgeon and S6 was the chief resident) at (A) extension, (B) midflexion, and (C) flexion.

Table 4.

Comparing the measured gaps in knee soft tissue assessment between five surgeons and a chief resident using RM-ANOVA

	Measured gap in extension (mm)		Measured gap in midflexion (mm)		Measured gap in flexion (mm)
Surgeon	Lateral	Medial	Lateral	Medial	Lateral	Medial
Group (S1) n = 6	1.6 ± 0.8	1.8 ± 0.5	1.6 ± 1.0	1.5 ± 0.7	1.6 ± 1.2	1.5 ± 0.7
Group (S2) n = 7	1.4 ± 0.8	1.7 ± 0.6	1.6 ± 0.6	2.0 ± 0.4	1.3 ± 0.8	1.6 ± 0.7
Group (S3) n = 6	1.3 ± 0.9	1.8 ± 0.4	1.5 ± 0.8	2.2 ± 0.8	1.4 ± 1.4	1.3 ± 0.9
Group (S4) n = 7	1.8 ± 1.0	2.1 ± 0.4	1.8 ± 0.8	1.8 ± 1.0	1.2 ± 0.8	1.2 ± 1.0
Group (S5) n = 7	2.1 ± 0.8	2.4 ± 0.5	2.4 ± 1.4	2.2 ± 0.9	2.0 ± 1.3	1.9 ± 0.8
Group (S6) n = 6	2.0 ± 1.0	2.0 ± 0.8	1.7 ± 0.9	2.3 ± 1.1	2.3 ± 1.9	2.0 ± 0.9
Mean ± SD	1.7 ± 0.3	2.0 ± 0.2	1.8 ± 0.3	2.0 ± 0.3	1.6 ± 0.4	1.6 ± 0.3
95% CI	(1.6-1.8)	(1.9-2.1)	(1.7-1.9)	(1.9-2.1)	(1.5-1.7)	(1.5-1.7)
p value	< 0.001	0.007	0.12	0.003	< 0.001	< 0.001

S1 to S6: Surgeon 1 to Surgeon 6 (S6 is the chief resident); mean = mean applied moments.

Weak Association Between Applied Moments and Insert Choice

The insert thickness choice was weakly and negatively associated with the magnitude of the varus (β = -0.06 ± 0.02 [95% CI -0.11 to -0.01]; p = 0.03) and valgus (β = -0.09 ± 0.03 [95% CI -0.18 to -0.01]; p = 0.03) moments in extension and the varus moment in flexion (β = -0.11 ± 0.04 [95% CI -0.22 to 0.00]; p = 0.04) (Table 5). To put this result in context, the largest regression coefficient (β = -0.11) indicates that increasing the applied varus moment in flexion by 9-Nm may result in selecting a 1 mm thinner insert. In all but one cadaver knee, the choice of insert thickness varied by l mm among surgeons (Table 2). In the remaining knee, it varied by 2 mm. This difference of insert choice was perceived by the surgeons who participated in this study and could alter ligament tension and the resulting medial and lateral gaps [4, 28].

Table 5.

Regression analysis using a generalized estimating equation to test the association between the applied moments by surgeons in extension, midflexion, and flexion and the choice of insert thickness

Knee flexion	Moment in Nm	Regression coefficient in mm/Nm (95% CI)	p value
Extension	Varusa	-0.06 (-0.11 to -0.01)	0.03a
	Valgusa	-0.09 (-0.18 to -0.01)	0.03a
Midflexion	Varus	-0.03 (-0.11 to 0.06)	0.45
	Valgus	-0.03 (-0.11 to 0.06)	0.47
Flexion	Varusa	-0.11 (-0.22 to 0.00)	0.04a
	Valgus	0.01 (-0.06 to 0.07)	0.80

^aSignificant association; the greatest regression coefficient (β = -0.11) indicates that for every 9-Nm increase in the applied varus moment (22 N of force applied to the foot assuming a shank length of 0.4 m), the choice of insert thickness decreased by 1 mm.

Discussion

Balancing soft tissue tension by assessing joint gaps is a critical aspect of achieving knee stability in TKA [19, 38]. Currently, most TKA surgeons rely on their intuition, experience, and training to assess soft tissue balance using a manually applied, subjective exam [6, 17]. Moreover, most tools used to aid intraoperative decision making in TKA, like surgical navigation and intraarticular force sensors, do not account for the magnitude of loads applied by the surgeon during gap assessment; thus, interpretation of these outputs may be confounded by variations in applied load. Unfortunately, the magnitude of loads applied by surgeons during intraoperative examination of the knee is unknown as is whether variations in the applied load are related to differences in surgical decisions such as the choice of insert thickness. Determining whether such relationships exist would justify whether and to what extent instruments that aid in applying consistent standardized loads need to be coupled with the aforementioned intraoperative tools. Our most important finding was that the five surgeons and a chief resident who participated in this study applied different varus and valgus moments when assessing knee balance in midflexion and flexion (Fig. 3). The applied moments ranged between 8.9 Nm and 13.3 Nm, which is equivalent to the surgeon applying 22.3 to 33.3 N of force to the foot assuming the length of the shank is 0.4 m (Table 3). Similarly, the measured lateral and medial gaps differed in extension, midflexion, and flexion between surgeons, with some surgeons accepting more lateral gapping in flexion (Fig. 4). The choice of insert thickness varied among surgeons by up to 2 mm, but it differed by 1 mm in most cases (Table 2). The choice of insert thickness was associated with the applied moments in varus and valgus in extension and in varus in flexion, albeit weakly in all cases. Importantly, our study design minimized the potential variations in surgical decisions compared with real in vivo surgery since insert thickness was the only allowed choice (the femoral component was fixed, and no ligament releases were performed). Nonetheless, relationships between insert choice and applied loads emerged in extension and flexion. Specifically, changes in the applied moments of 9 and 11 Nm (22 to 27 N on the surgeons’ hands) in flexion and extension, respectively, yielded change in choice of insert thickness by 1 mm or more. Thus, our findings suggest that systems aiming to standardize and guide TKA decision-making should integrate at least low-resolution force measurement to control for the loads applied during intraoperative knee examination. More specifically, in our example, a force sensor capable of detecting forces of at most 22 N would help to limit variation in choice of insert thickness to 1 mm or less.

Limitations

The study has limitations. First, it was conducted using nonarthritic human cadavers. The soft tissue properties of arthritic knees could be stiffer than in the nonarthritic condition [5]. Thus, variations in insert thickness would cause greater changes in soft tissue tension. For example, based on the average stiffness of the superficial medial collateral ligament (MCL) (81 N/mm) in the nonarthritic knee, a 1-mm change in MCL length (due to variations in insert thickness of 1-mm) would change MCL force by 81 N [28]. Increased MCL stiffness due to osteoarthritis would increase the sensitivity of MCL force to changes in insert thickness. Since 45 N of MCL tension has been shown to achieve clinically acceptable extension gaps less than 1 mm [4], changes in MCL force imparted by a 1-mm change in insert thickness could alter surgeon perceptions of soft tissue balance. Thus, our findings in nonarthritic knees are likely a lower bound, which probably minimized the variability in choice of insert thickness among surgeons. Second, the knee ligaments could have stretched after repeated testing. However, we confirmed that the insert choice of the first surgeon did not change when he repeated his assessment after all other surgeons had completed their testing. Third, the surgeons only chose the insert thickness. Surgeons can make other decisions to balance the knee such as ligament releases and bony recuts [6, 38]. Since the number of surgical decisions was limited, our study design likely minimized the variability among surgeons. Even so, we found differences in the applied moments, assessed gaps, and choice of insert thickness. Thus, our findings are possibly a lower bound of the variability encountered in the less-controlled in vivo intraoperative setting. The novel methods presented in this study could be used in subsequent in vitro or intraoperative studies to quantify the influence of other methods to balance the knee on surgical decision-making. Fourth, surgeons were instructed to examine the knee at approximately 0°, 30°, and 90° of flexion. However, the actual angles were surgeon-dependent, which reflects how the examination is done in the operating room. Fifth, we measured the applied forces and knee gaps only during the final knee balance assessment with the chosen insert. This eliminates the mental data gathering of the surgeon during the process of knee balance. Therefore, surgeons could become biased and only apply the amount of force necessary to confirm their decision. Nonetheless, we found high variability in the applied forces and measured gaps among surgeons even in their final assessment; thus, we believe our main conclusion would still hold.

Sixth, we intentionally did not control hip rotation during intraoperative trialing to allow gap assessment in our cadaver model to better simulate the operating room. Different hip rotations may alter the tension of tissues that contribute to knee resistance to external loads, such as the iliotibial band [36]. Thus, different hip rotations may cause surgeons to apply more or less varus/valgus moments during their trialing at midflexion and flexion. Seventh, we used a fixed-bearing posterior cruciate ligament substituting implant system that features a side-specific tibial tray, tibial insert, and femoral component. This implant system was chosen based on the preference and experience of the operating surgeon (PKS). The implant design removed any influence that the cruciate ligaments had on the assessments of varus-valgus stability at the three flexion angles that were examined. Thus, our results cannot be generalized to implant systems that preserve one or both cruciate ligaments. Nevertheless, our findings are likely generalizable to condylar, posterior cruciate ligament–substituting TKA systems with relatively nonconforming bearing surfaces, which comprise a large percentage of TKA systems employed in the United States [24]. Eighth, mechanical cutting jigs were used in bone resections and neither robotic assistance nor computer navigation was used for the cadaveric surgery. However, this method reflects the most common clinical scenario. Ninth, both male and female cadavers were included in the study. The magnitude of applied moments, measured gaps, and choice of insert thickness were similar across sexes, suggesting that our findings depended more on the surgeon than the sex of the cadaver. Finally, the five surgeons who participated in this study were trained at or practice in the same institution. Thus, a multi-institutional study is warranted to confirm the generalizability of our findings.

Applied Moments Differed Among Surgeons

Among the surgeons participating in our study, we observed differences in the applied moments in the trialing exams in midflexion and flexion in both varus and valgus (Table 3). This finding is important for three reasons: First, the result demonstrates the extent of variation among surgeons in determining the level of tension that they must perceive to evaluate the medial and lateral gaps in midflexion and flexion. Second, the result highlights the importance of controlling the input loads in the trialing exam when using balancing tools, such as instrumented tibial trial and navigation systems [17, 37]. Specifically, the output measures of these instruments depend on how each surgeon loads the knee in varus and valgus [33]. For example, a 10 Nm increase in the applied valgus moment could change knee valgus rotation by 3° to 5° [33]. Therefore, surgeons using these tools must be aware that they need to apply consistent loads to standardize the output measures of their exams. Third, in extension, the applied valgus moments were more uniform (p = 0.60) than the applied varus moments among the participating surgeons (Table 3). This finding corroborated observations from our surgical team about the ability to consistently feel a distinct, firm endpoint under valgus loading compared with varus loading since the medial collateral ligament is stiffer than the lateral collateral ligament [16, 29]. Thus, surgeons might be able to target a consistent valgus moment more easily.

Measured Gaps Differed Among Surgeons

The lateral and medial gaps measured via motion capture differed among surgeons (Fig. 4). This finding corroborates previous studies reporting that surgeons target different gaps [13, 20]. The choice of insert thickness was within 1 mm in all cases but one (Table 2). This finding indicates that some surgeons accept larger gaps or that the selected implant thickness achieved the best compromise in gap balance for all tested flexion angles. Interestingly, the lateral gap in flexion produced the largest difference among surgeons (Table 4). Previous studies suggested that a large lateral gap better reproduces natural kinematics by enabling rollback of the lateral femur [15, 27, 30]. Thus, some surgeons in this study may have been more comfortable accepting a larger lateral gap in flexion. This study was not powered to assess the relationship between measured gaps and insert choice. However, this work provides the data needed to power a future study to determine whether such an association exists.

Negative Association Between Applied Moments and Insert Choice

The choice of insert thickness was negatively and weakly associated with the applied moments in varus and valgus in extension and varus in flexion (Table 5). The largest regression coefficient (β) was -0.11 mm/Nm in varus in flexion. A regression coefficient of -0.11 mm/Nm means that for every 9-Nm increase in the applied varus moment (that is, 22 N of force applied to the foot assuming a shank length of 0.4 m), the choice of insert thickness decreased by 1 mm. Therefore, a force sensor capable of detecting forces of at most 22 N would help to limit variation in choice of insert thickness to 1 mm or less. A relationship existed despite the limited number of choices allowed in our experimental design focusing on selection of insert thickness with a fixed femoral component and tibial tray. Our findings suggest that systems aiming to standardize and guide TKA decision-making, via navigated gap assessment and intraarticular force measurement, should consider integrating measurement of the applied loads to standardize this intraoperative examination. We would expect variations in choice of insert thickness to be greater if the surgical choices were less constrained; therefore, our suggestions represent minimum requirements. The negative association could indicate that some surgeons prefer a more lax knee and may apply more force but choose a thinner insert and vice versa. To further quantify how the surgeon arrives at their decision, future work using the methods developed in this study could quantify the applied moments while trialing different insert thicknesses to understand the mental data gathering for each surgeon during the process of knee balancing.

Conclusion

In a human cadaver model of soft tissue assessment in TKA, five arthroplasty surgeons with different levels of experience and a chief resident applied different magnitudes of varus and valgus moments in midflexion and flexion and targeted different gaps in extension, midflexion, and flexion. The largest difference in the mean applied moment was in varus in midflexion, and the largest difference in the mean gap was the lateral gap in flexion. A weak and negative association between the choice of insert thickness and applied loads emerged in extension and in flexion. These associations revealed that differences in the applied moments of 9 and 11 Nm in flexion and extension, respectively (22 to 27 N on the surgeon’s hand), yielded differences among surgeons in the choice of insert thickness by 1 mm or more. The choice of insert thickness may be more sensitive to the applied moments in actual in vivo surgery since the surgeon is allowed a greater array of choices beyond insert thickness. Therefore, systems aiming to standardize and guide intraoperative decision-making, via navigated gap assessment and intraarticular force measurement, should consider integrating measurement of the applied loads to better control for variations in insert selection among surgeons in TKA.