Abstract

During total knee arthroplasty (TKA) surgery, an important step is determining the correct insert thickness for each patient. If the insert is too thick, then stiffness results. Or conversely, if the insert is too thin, then instability results. One common method used to determine the insert thickness is by manually assessing the joint laxity; this is a qualitative method that depends on the surgeon's experience and “feel” and is unreliable. The lack of objective methods to reliably determine the correct insert thickness creates a need to develop such a method. One possible method is to measure the force required to push a trial insert into position, requiring a specialized tool to measure the push force. Hence, a new measuring tool was designed to measure the push force intra-operatively, accurately, and safely. To demonstrate functionality, the tool was tested on three patients. During the tests, the surgeon determined the appropriate thicknesses of the insert to trial and proceeded to position three different insert thicknesses ranging from 10 mm to 12 mm, and example push forces were recorded. The new tool met all the design criteria, and the example results from the patient testing show potential in using the peak push force to identify the insert with the correct thickness.

Introduction

During total knee arthroplasty (TKA) surgery, an important step is determining the correct insert thickness for each patient. On the one hand, if the insert is too thick, then this leads to stiffness, decreased range of motion, and high tibial compartment forces [1,2]. On the other hand, if the insert is too thin, then this leads to instability [2,3].

One common method to determine the insert thickness is by manually assessing the joint laxity, particularly in varus-valgus rotation. However, this is a qualitative method that depends on the surgeon's experience and “feel,” which is not reliable [4,5]. Additionally, research shows that selecting the “correct” tibial insert thickness by manually assessing the varus-valgus laxity fails to detect an insert that is too thick, resulting in a stiff knee [6].

As a result of the qualitative nature of manually assessing the laxities and the concomitant unreliability, several studies have tried using tibial force sensors to help determine the correct insert thickness [1,5,79]. However, these sensors are costly, cannot be reused, and are unavailable for some tibial insert designs. The lack of a cost-effective method generally applicable for an arbitrary insert design to determine the correct insert thickness highlights the need to develop such a method.

A conceptually new method is to measure the peak force required to push a trial insert into position. The rationale behind such a method is traced to the load–displacement behavior of the tibiofemoral joint in flexion under an applied distraction force. With the knee in flexion, the load–displacement behavior is characterized by a relatively large displacement for a 50  N distraction force followed by minimal increase in displacement with increasing force [10]. Hence, for an insert that is too thick, the change in peak push force (i.e., slope) for a thicker insert than correct would be expected to be considerably greater than the change in peak push force for a thinner insert than correct. Accordingly, a marked increase in slope as insert thickness is increased might be used to detect an insert which is too thick in which case the next thinner thickness would be correct. To critically evaluate the workability of this new method, a specialized tool that can measure the push force is required.

This tool must meet several design criteria. One is that it should be reusable, which means it must be sterilizable. Related is that a tool with multiple components must be disassembled to meet operating room sterilization guidelines. When components must be reassembled in the sterile operating field, the reassembly should require a minimum number of conventional tools. Material for the tool must be corrosion-resistant. Finally, the shape and weight should replicate the current tool and contain a load cell that measures the push force applied by the surgeon to position the trial insert between the femoral and tibial components.

In a similar manner, the load cell used to measure the push force must meet several design criteria. First, the load cell must be sterilizable without damaging the electronics. Second, it must quantify a push force up to 225 N, which is the maximum estimated push force that can be applied manually. Finally, it must accurately measure the push force in real-time.

The objectives were threefold. One was to describe the design of a new surgical tool which measures the push force intra-operatively. A second was to demonstrate functionality by using the tool intra-operatively to generate push force data for inserts of different thickness on a sample of several patients. A third was to process push force data to assess the potential of using the peak push force to identify the correct insert thickness.

Methods

Design of Handheld Push Force Measuring Tool.

The push force measuring tool was designed using SOLIDWORKS, a computer-aided design software package. The design comprises a handle, fork, set screw, ball-pin detent, and load cell (Fig. 1). The ball-pin detent is used to attach the fork to the handle. Because the handle freely translates along the shaft of the ball-pin detent, the push force is transmitted only through the load cell. The set screw, which engages a slot in the ball-pin detent shaft, prevents rotation of the handle relative to the shaft. The ball-pin detent consists of a hollow shaft that contains a spindle and spring, which hold the balls in place unless the button is pressed (Fig. 2). The handle, fork, and ball-pin detent are made from 304 stainless steel, making the tool sterilizable with conventional methods such as autoclaving. Other than sliding the ball-pin detent through the handle, load cell, and into the fork, the only assembly step is to mount a set screw that uses a conventional 2 mm stainless steel hex key that allows it to be assembled in the operating theater.

Fig. 1
Rendering of push force measuring tool showing the various components disassembled (top) and assembled (bottom)
Fig. 1
Rendering of push force measuring tool showing the various components disassembled (top) and assembled (bottom)
Close modal
Fig. 2
Rendering of ball-pin detent in cutaway
Fig. 2
Rendering of ball-pin detent in cutaway
Close modal

Data Acquisition Set-Up.

Push force data were acquired using a load cell, bridge amplifier, and data acquisition system. A 222.5 N through-hole load cell (MBD2 Through-Hole Force Sensor, Flintec, Hudson, MA) was used to measure the push force. The load cell was connected to a NI 9237 Bridge Analog Input inserted into a cDAQ-9174 Compact DAQ Chassis Module (National Instruments, Austin, TX) connected to a computer via USB port. The data were collected at 25 kHz using NI DAQExpress software version 5.1.

Testing of Handheld Push Force Measuring Tool.

After obtaining approval from the University of California Davis Institutional Review Board (No. 1896740) and patient informed consent, the functionality of the sterilized tool was tested on three patients undergoing TKA surgery. The inclusion criteria were patients capable of consenting and undergoing TKA. Exclusion criteria were evidence of a fracture in either limb, history of neurological disorder, and history of rheumatoid or post-traumatic knee arthritis.

The central processing department in the hospital sterilized the load cell and cable using vaporized hydrogen peroxide gas plasma technology (STERRAD 100NX) to preserve the function of the electronics. An autoclave sterilized the handle, fork, and ball-pin detent.

The surgeon performed caliper-verified, unrestricted kinematically aligned (unKA) TKA [11] using kinematic alignment specific components consisting of an insert with medial ball-in-socket conformity, flat lateral articular surface, and posterior cruciate ligament retention (SpheriKA, Medacta, Castel San Pietro, Switzerland). Unrestricted KA TKA is a personalized approach in which the resections are made to restore the pre-arthritic, native alignments of the limb and joint lines without collateral ligament release. Using manual instruments, this technique restores native alignments with high accuracy [12] and balances the knee as evidenced by restoration of tibial contact forces which match native [6,13]. As part of the normal surgical procedure, the surgeon determined the appropriate thicknesses of the insert to trial from an assessment of the resection surfaces using a spacer block and proceeded to position three different insert thicknesses in 1 mm increments ranging from 10 mm to 12 mm (Fig. 3). Using an insert goniometer, which indicated the internal–external tibial rotation as the knee was flexed passively to 90 deg [14], the correct insert thickness was that which maximized internal tibial rotation and was the middle thickness for each patient. The push force to position the inserts between the femoral and tibial components was recorded in real-time for each thickness.

Fig. 3
Surgeon using the handheld push force measuring tool. Surgeon with trial insert attached to tool (left). Surgeon positioning trial insert between femoral and tibial components (right).
Fig. 3
Surgeon using the handheld push force measuring tool. Surgeon with trial insert attached to tool (left). Surgeon positioning trial insert between femoral and tibial components (right).
Close modal

After acquisition, the data were exported and analyzed using MATLAB. Peak push forces were recorded. The peak push forces for each patient were normalized to the value for the middle insert thickness and plotted versus the insert thickness.

Results

The push force records were distinguished by two peaks, with the first peak being greater in magnitude than the second (Fig. 4). Focusing on the first peak, the peak push forces for two of the three patients showed a monotonic increase as the insert thickness increased (Table 1), with a marked increase in slope as the insert thickness increased from the middle insert thickness to the 1 mm thicker insert (Fig. 5).

Fig. 4
Example push force versus time graphs for three insert thicknesses of 10 mm, 11 mm, and 12 mm for a patient
Fig. 4
Example push force versus time graphs for three insert thicknesses of 10 mm, 11 mm, and 12 mm for a patient
Close modal
Fig. 5
Normalized peak push force versus insert thickness for three patients
Fig. 5
Normalized peak push force versus insert thickness for three patients
Close modal
Table 1

Insert thickness, peak push force, and corresponding normalized peak push forces for three patients

Insert thickness (mm)Peak push force (N)Normalized peak push force
Patient 1101110.95
111171.00
123713.17
Patient 2102341.00
112321.00
121870.81
Patient 3101050.99
111071.00
121411.32
Insert thickness (mm)Peak push force (N)Normalized peak push force
Patient 1101110.95
111171.00
123713.17
Patient 2102341.00
112321.00
121870.81
Patient 3101050.99
111071.00
121411.32

Discussion

The design met the main challenge of repeat sterilization of the load cell before each surgery. This proved challenging because most load cells with the proper dimensions and load rating could not tolerate the temperature and humidity of the autoclave process. Other possible methods to sterilize sensitive electronic equipment, such as ethylene oxide, were explored; however, it is flammable, reactive, and carcinogenic [15]. As a result, vaporized hydrogen peroxide gas plasma technology was used as an alternative because it avoided these difficulties.

Another challenging aspect was to attach a load cell to the handheld tool without interfering with load transmission. To meet this challenge, the tool was designed as a prismatic joint, which only allows translation along the handle. As a result of this design, when the handle is used, it slides freely along the shaft of the ball-pin detent, allowing all the push force to go through the load cell.

Manufacturing the push force measuring tool also proved challenging. The small size, stainless steel material, and fork mechanism used to hold the trial insert required tight tolerances, which increased the difficulty of the machining process. Specific challenges were keeping the cutting tool properly cooled and sharp, which required machining at a slow rate to reduce the risk of damaging the cutting tools with the disadvantage of lengthening the production time.

In the final analysis, the new push force measuring tool met all the design criteria necessary to measure the push force accurately and safely. All components were successfully sterilized, including sensitive electronics such as the load cell. The dimension and weight of the push force measuring tool, based on the surgeon's feedback, are comparable to the previous tool and the long cable allowed the surgeon to have free movement during the surgery. The system consisting of the push force measuring tool in conjunction with the signal conditioning and data acquisition electronics was able to record the push force data and plot it in real-time. Importantly, the new push force measuring tool overcomes the disadvantages of tibial force sensors. Unlike tibial force sensors, the push force measuring tool is cost effective in that it is reusable and can be adapted to an arbitrary tibial insert design.

A few possible minor improvements for following iterations of the tool are to reduce the profile of the detent handle to improve the ergonomics of the tool's handle and move the button placement. Although the surgeon does not grip the detent handle, the T-shaped profile may interfere with the surgeon gripping the tool's handle. Additionally, the button's placement may lead to the surgeon accidentally releasing the fork while pushing the insert into place.

As noted in the Introduction, the premise of using the peak push force to indicate the correct insert thickness hinges on the behavior of the load–displacement relation when a distraction force is applied to the tibia with the knee flexed. In most directions of loading, load–displacement relations of the knee are characterized by an initial low stiffness or laxity region followed by a high stiffness region once soft tissue structures become engaged [16,17]. Such behavior occurs in distraction once the knee is flexed [10,18]. Hence, with the knee flexed, the change in peak push force (i.e., slope) for an insert thicker than correct would be expected to be considerably greater than the change in peak push force for an insert thinner than correct. This is because to push the thicker insert into position, the push force must overcome the resistance particularly of the collateral ligaments. Because these ligaments are stiff (i.e., ∼70 N/mm [19]), the change in push force necessarily would be large. Given the high stiffness and that a 1 mm difference in insert thickness can significantly affect load–displacement relations [3], many manufacturers offer a selection of inserts which vary in thickness by 1 mm.

Although sample results demonstrate that the slope increased markedly with increasing insert thickness for 2 of 3 patients, a number of factors could introduce variability in a larger sample of patients which might render the method not useful. For example, to push the insert into position, the knee must be flexed to about 15 deg. However, the flexion angle is not measured in practice and laxity varies with flexion angle [10,18,20]. Another factor is orientation of the insert when pushed into position. Most inserts have some degree of conformity particularly with the medial femoral condyle [12,21]. If the insert is rotated in flexion–extension so that the posterior lip is scooped underneath the medial femoral condyle to facilitate positioning, then this action could reduce the peak push force compared to a linear posterior movement which would require greater distraction of the medial compartment. Other factors that might introduce variability are the patient's age [22,23], sex [2325], and body mass index. Given these factors, to thoroughly determine whether the peak push force can be used to indicate the correct insert thickness, a clinical study must be conducted with a large sample of patients.

Conclusions

A new objective method to intra-operatively determine the correct insert thickness was conceived, a tool for measuring the push force of tibial inserts of different thicknesses was designed and fabricated, and intra-operative functionality was demonstrated through testing on several patients. By integrating a commercially available load cell into an existing tool, the push force measuring tool met all the design criteria. Example results show potential in using the peak push force to identify the insert with the correct thickness. If this potential were realized, then the method would offer an objective means to select the correct insert thickness thus reducing the possibility of adverse outcomes due to stiffness and loss of motion from an insert that is too thick and instability from an insert which is too thin.

Funding Data

  • Medacta USA, Inc.

Data Availability Statement

The authors attest that all data for this study are included in the paper.

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