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Original Research

Frontiers in Medical Engineering ·

Volume 4 - 2026 | https://doi.org/10.3389/fmede.2026.1548013

Impact energy during cadaveric transforaminal lumbar interbody fusion (TLIF) is replicated by a modular benchtop device

  • Caitlin Luke1
  • Micah Foster2
  • Alexis Graham2
  • Tanner Jones2
  • Halleigh Faulkner3
  • Alyna-Marie Janus2
  • Clark Hensley4
  • Jerald Redmond5
  • MeLeah A. Henson5
  • Lauren B. Priddy2
  • Matthew W. Priddy1*corresponding author
  1. 1Michael W. Hall School of Mechanical Engineering, Mississippi State University, Mississippi State, MS, United States
  2. 2Department of Agricultural and Biological Engineering, Mississippi State University, Mississippi State, MS, United States
  3. 3Department of Aerospace Engineering, Mississippi State University, Mississippi State, MS, United States
  4. 4Department of Computer Science and Engineering, Mississippi State University, Mississippi State, MS, United States
  5. 5Medtronic Spine and Biologics, Memphis, TN, United States
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Abstract

In the treatment of intervertebral disc pathologies, lumbar interbody fusion (LIF) involves the removal and replacement of the degenerative disc with an interbody fusion device (IFD). Leveraging a benchtop impaction device for repeatable replication of cadaveric IFD insertion would alleviate the challenges of relying solely on cadaveric models and accelerate the establishment of use condition parameters for refinement of IFD design and LIF procedures. Using a custom benchtop device, the objective of this work was to determine the benchtop testing conditions which mimic the impact energy absorption during cadaveric transforaminal lumbar interbody fusion (TLIF). From initial experiments with 1.0 lb (0.454 kg) drop weights of steel, zinc, and aluminum, we determined aluminum facilitated an impact duration closest to that of historical cadaveric TLIF data. Thus, subsequent testing utilized the aluminum drop weight impacting Ti-6Al-4V IFDs (12 mm height x 24 mm length). A lateral compressive load of 200 N was applied using springs with a stiffness of 63.22 lb/in (11.07 N/mm). Drop heights from which the aluminum weight was dropped ranged from 60 cm to 120 cm in increments of 10 cm. Data were collected by an impact force sensor on the IFD insertion instrument, button compression sensors on the compression platens, and a laser displacement sensor below the IFD. The impact waveform for benchtop and cadaveric data was characterized by four waveform characteristics: impact duration, area under the impulse curve, peak force, and initial impact slope of the waveform. Area under the curve represented energy absorption from the insertion instrument to the IFD. The cadaveric impact duration was replicated by all drop height groups on the benchtop device, and the peak impact force was replicated by the 90 cm, 110 cm, and 120 cm groups. The area under the impulse curve was replicated by the 110 cm and 120 cm groups. No benchtop group replicated the initial impact slope of the waveform seen in cadaveric data. In conclusion, a benchtop impaction device was validated for replication of energy absorption during insertion of IFDs in cadaveric TLIF procedures. Ultimately, this work will accelerate advancements in IFD design and failure analysis, enhancing the repeatability of TLIF surgical techniques.

Keywords

1 Introduction

Impaired disc function often causes lower back pain due to nerve pinching, spine weakness, and structural failure (Adams and Roughley, 2006). Lumbar interbody fusion (LIF) is the primary surgical treatment for spinal disc pathologies such as degenerative disc disease, spondylolisthesis, and disc herniation (Verma et al., 2020). Further, low back pain is the second most prevalent cause of disability for adults in the United States (Garg and Mehta, 2019). Transforaminal lumbar interbody fusion (TLIF) is the most commonly employed LIF procedure, as the posterolateral exposure minimizes neural retraction, utilizes a posterior unilateral surgical approach, and avoids midline scarring (Garg and Mehta, 2019; Verma et al., 2020). During TLIF procedures, the compromised intervertebral disc is removed and replaced with an interbody fusion device (IFD) (Verma et al., 2020). This treatment option effectively restores spinal alignment, lordosis, and height of the intervertebral disc space; stabilizes the spine; provides neural decompression; and promotes spinal fusion via osseointegration (Garg and Mehta, 2019; Verma et al., 2020).

IFDs are commonly manufactured from titanium alloys and polyetheretherketone (PEEK) (Mobbs et al., 2016; Verma et al., 2020). The titanium alloy devices are commonly designed with porous (lattice) structures to reduce stiffness, prevent subsidence, and provide greater energy absorption than solid structures (Fogel et al., 2022; Wang et al., 2023; Yin et al., 2023). To ensure spinal alignment and restoration of disc space height, the size and shape of the IFD is chosen based on the patient’s anatomy. The IFD is inserted using a LIF process whereby the IFD is attached to the end of an insertion instrument and inserted into the disc space via malleting of the instrument until the IFD is fully seated between the vertebrae (Palepu, 2019; Palepu et al., 2021). While this method of impaction for IFD insertion is successful in the majority of cases, on rare occasions it has resulted in intra-operative device failure (FDA, 2015a; FDA, 2017a; FDA, 2017b; U.S. Food and Drug Administration, 2021), which may be due to impaction forces exceeding the expected forces for which the IFD was designed (FDA, 2017a; FDA, 2017b; 2015b; U.S. Food and Drug Administration, 2022; U.S. Food and Drug Administration, 2021; Palepu et al., 2021; Peck et al., 2018). Failures involving the insertion instrument have also been reported (U.S. Food and Drug Administration, 2021; FDA, 2015a; U.S. Food and Drug Administration, 2025).

Traditionally, cadavers have been utilized in medical education to practice surgical procedures and serve as a surrogate to quantify the spine’s biomechanical response to loading conditions (Duda et al., 2007; Pacca et al., 2017; Proano et al., 2002; Spitzer and Whitlock, 1998; Thelen et al., 2012). Cadavers have frequently been used in the development of IFDs and refinement of LIF procedures (Alkalay et al., 2018; Shasti et al., 2019). Despite being necessary for training, as well as validation and refinement of surgical procedures, cadaveric models present disadvantages. Cadaveric testing introduces variation in data and testing conditions due to variances in anatomies and pathologies among cadavers, as well as variability in procedures among surgeons and subject matter experts (SMEs). Cadavers are also limited in availability and require a significant amount of time for preparation (Eizenberg, 2015; Tai et al., 2018). Nonetheless, cadaveric testing mimics the surgical procedure (Alkalay et al., 2018), and the energy absorption measured during cadaveric testing is expected to be an accurate representation of TLIF surgical procedures. Thus, the ability to quantify the energy absorbed by the IFD during insertion is valuable design data for reducing the incidence of IFD failure. Leveraging a benchtop model designed to replicate cadaveric TLIF impaction would alleviate the challenges of relying solely on cadaveric models and accelerate the establishment of use condition parameters for IFD design and refinement of LIF procedures (Graham et al., 2023; Palepu et al., 2021; Peck et al., 2018).

Since the IFD must withstand significant impact forces during insertion, quantification of energy absorption during the insertion process is important for product design and material selection (Lu and Yu, 2003). While data on the force experienced by the insertion instruments is scarce, some studies have utilized a benchtop device to examine the effects of the insertion process on IFDs (Kienle et al., 2016; Torstrick et al., 2018). We and others have characterized the impact event of insertion instrument malleting in a cadaveric TLIF procedure and mimicked the event on a benchtop device (Graham et al., 2023; Palepu et al., 2021). Palepu et al. evaluated PEEK IFDs and only compared the impact duration (area under the force-time curve) between cadaveric and benchtop data (Palepu et al., 2021). In our previous work, a custom benchtop device successfully replicated cadaveric peak impact force and initial impact slope of the waveform (Graham et al., 2023). We previously reported impaction data from cadaveric TLIF testing using a modified insertion instrument outfitted with a force sensor, and compared that data with data from a custom-built benchtop device with the same instrument-sensor configuration, including the effects of varying IFD height (size), drop height, and drop weight mass (Graham et al., 2023). While this work validated the use of a sensor-enabled insertion instrument to replicate peak impact force and the initial slope of the impact waveform (Graham et al., 2023), improvements to the benchtop device were necessary to more thoroughly mimic the impact energies observed during cadaveric TLIF procedures. Successful replication of other waveform characteristics including impact duration and area under the impulse curve from cadaveric TLIF procedures have yet to be achieved in a benchtop model. Quantification of these characteristics of the impact waveform is vital for refinement of a benchtop device that can accurately simulate the energy absorption (i.e., area under the impulse curve) experienced by the IFD upon impact during a TLIF procedure (Lu and Yu, 2003). Replication of energy absorption from cadaveric experiments will validate the utility of the benchtop device as a surrogate for cadaveric testing and inform future studies to better characterize and predict IFD failure. This research gap necessitates the refinement of benchtop devices to more accurately model cadaveric loading and a more robust cadaver-to-benchtop comparison for impact event characterization through the analysis of waveform characteristics.

Thus, the objectives of this work were: (i) to quantify the energy absorption during IFD impaction in cadaveric procedures, and (ii) to refine a benchtop device to enable mimicking of four cadaveric loading waveform characteristics: impact duration, area under the impulse curve, peak impact force, and initial impact slope. Because aluminum possesses a higher coefficient of restitution than steel, it was hypothesized that the use of an aluminum drop weight at drop heights ranging from 60 cm to 120 cm would result in impact duration, area under the impulse curve, peak impact force, and initial impact slope similar to those from cadaveric TLIF.

2 Materials and methods

2.1 Modifications to benchtop testing device

A benchtop device developed previously (Graham et al., 2023) was modified for this work to provide a more controlled testing environment for replicating cadaveric data (Figure 1). Compression clamps (Labor-Saving Energy-Saving Arm, Item No. 0221, XClifes) were incorporated into the benchtop device to facilitate separation of the platens for IFD seating prior to testing and IFD removal between tests (Figure 2A). Two FUTEK® button compression sensors (LLB300 Miniature Load Button, Item No. FSH03955, FUTEK, Irvine, California, United States) with a load range of 25–1,000 lbs [11.34–453.6 kg] were positioned behind the springs (spring constant of 63.22 lb/in (9657K12, McMaster-Carr, Atlanta, GA, United States)) and used to measure the lateral compressive load. Spring guides were machined from steel rods to effectively distribute the compressive force to the sensors. Frictionless rails were utilized on the bottom of the benchtop device, which allowed the attached compression platens to move freely. A baseplate was incorporated into this design to serve as the mount for the frictionless rails and the laser displacement sensor (Q4X Laser Distance Sensor, Banner Engineering Corp., Minneapolis, United States). Hex head screws (1/2″-13 thread size, 6″ long) and L-brackets were added to the t-frame to aid in equal distribution of compressive forces, keep the platens parallel, and prevent the springs from buckling.

CAD images of the modifications made to the previous benchtop device design (Graham et al., 2023). In **(A)**, the sensor modifications from the previous benchtop design are italicized and bolded. All components mentioned in **(B)** are modifications from the original design.
Figure 1. CAD images of the modifications made to the previous benchtop device design (Graham et al., 2023). In (A), the sensor modifications from the previous benchtop design are italicized and bolded. All components mentioned in (B) are modifications from the original design.
**(A)** Compression clamps on benchtop device. **(B)** CAD model of wire adapter and housing piece for the impact sensor.
Figure 2. (A) Compression clamps on benchtop device. (B) CAD model of wire adapter and housing piece for the impact sensor.

A guide for setting the initial placement of the insertion instrument was designed and 3D printed to facilitate seating of the IFD at a consistent depth of 21.3 mm between the platens prior to each insertion. The lateral spacing of the compression platens was 12 mm, consistent with the height of the implant. A support piece was incorporated into the 3D printed wire adapter to provide additional support to the impact force sensor wiring connection (Figure 2B). The wire adapter fits over the impact force sensor, and an additional housing piece encloses the sides while leaving the impact surface exposed. The impact force sensor was attached to the wire adapter via a hose clamp to ensure a stable connection. The goal of each modification was to minimize both variance in results and time required for setup and testing.

2.2 Waveform characteristics

Figure 3 depicts a typical impact force waveform. From this waveform, all outcome measures including impact duration, area under the impulse curve, peak impact force, and initial impact slope are extracted via a Python script.

Impact force waveform with impact duration, area under the impulse curve, peak impact force, and initial impact slope.
Figure 3. Impact force waveform with impact duration, area under the impulse curve, peak impact force, and initial impact slope.

All waveform characteristics are defined in the same way as in our previous work (Graham et al., 2023). The threshold for the beginning and end of the impact waveform was 1.5 V, as measured by a piezoelectric impact force sensor (PCB 208C05, PCB Piezotronics, Depew, United States). The impact force sensor’s sensitivity was 1.0 mV/lb (224.82 mV/kN), and its load limit was 5,000 lb (22.24 kN). The impact duration was defined as the time over which the waveform was above the 1.5 V initial threshold. The area under the impulse curve was approximated using the left Reimann sum of the waveform data above the 1.5 V threshold. Peak impact force was defined as the maximum force after the 1.5 V threshold. Initial impact slope was defined as the slope of the first rise after surpassing the 1.5 V threshold. Based on Chauvenet’s criterion, data points that were less than zero or greater than 35 kN for peak impact force (upper limit of sensor) were classified as partial strikes and removed from the dataset to minimize outliers in the data (Sherman, 2025).

2.3 Drop weight material selection

In our prior work, low-carbon steel 1018 drop weights were used, which exhibited a shorter impact duration than that from cadaveric data (Graham et al., 2023). It was hypothesized that a weight with a higher coefficient of restitution than steel would increase the impact duration, more effectively replicating impact duration from cadaveric loading. The coefficient of restitution (COR) is a value used to estimate the amount of energy dissipated during an impact event (Ahmad et al., 2016). COR can range from zero to one, where zero represents a perfectly plastic impact, and one represents a perfectly elastic impact (Ahmad et al., 2016). When the COR value is closer to one, the larger change in relative velocity results in larger impulses (Ahmad et al., 2016; Vernier, 2011). Therefore, selecting a drop weight material such as zinc or aluminum with a higher COR is expected to result in a prolonged impact duration.

The COR was calculated for low-carbon steel 1018 (8920K271, McMaster-Carr, Atlanta, GA, United States), commercially pure (CP) zinc (5048N151, McMaster-Carr, Atlanta, GA, United States), and 6061 aluminum (8975K444, McMaster-Carr, Atlanta, GA, United States) using their respective mechanical properties as well as Equations 1, 2, where ρ is the density, V i is the impact velocity, σ y is the yield strength, and E l * is the effective Young’s modulus (AISI 1018 Steel, cold drawn, 2023; ASM Material Data Sheet, 2023; Makeitfrom, 2023). These equations predict the COR of an elastoplastic granule when impacting a flat surface (Mangwandi et al., 2007). The COR calculated by Equation 1 (C O R p l a s t i c) assumes full plasticity during the loading stage (Johnson, 1987; Mangwandi et al., 2007). The COR calculated by Equation 2 (C O R e l a s t i c) accounts for the elastic contribution during loading (Mangwandi et al., 2007; Thornton, 1997). Additionally, these equations assume the impact is between like materials (Mangwandi et al., 2007).

C O R p l a s t i c = 1.72 σ y 5 E l * 4 ρ 1 8 V i − 1 4 (1)

C O R e l a s t i c = 1.324 σ y 5 E l * 4 ρ 1 8 V i − 1 4 (2)

One-pound cylindrical drop weights were machined from rods of each of the three materials. Each weight was 31.75 mm in diameter and varied in length to achieve the correct weight with a tolerance of ±0.02 lbs. For this testing, three benchtop insertions were performed with each drop weight material (n = 3). Full insertion was defined as 1.5 times the length of the IFD (Graham et al., 2023). All insertions used a drop height of 60 cm, a lateral compressive load of 200 N, and a 12 mm × 24 mm (h x w) Ti-6Al-4V IFD provided by Medtronic, Inc. (Memphis, TN, United States).

2.4 Spring stiffness and lateral compressive load

Spinal pathologies, such as degenerative disc disease, often result in increased stiffness of the spine motion segment (MS) (Brown et al., 2002). The springs used to apply the compressive load in the benchtop device were selected to mimic cadaveric stiffness of the lumbar spine while in a prone position (Owens et al., 2007). Previous work reported an average spinal stiffness of 10.8 N/mm (approximately 62 lb/in) for patients with low back pain lying prone (Owens et al., 2007). Based on this spinal stiffness, springs with a stiffness of 63.22 lb/in were used in the benchtop device.

In addition to mimicking the stiffness of the lumbar spine, the compressive force of the vertebrae on the IFD was also simulated. A compressive load of 200 N was selected based on a study by Palepu et al. utilizing a similar device and a 200 N preload (Palepu et al., 2021), which was based on the findings of Kienle et al. (Kienle et al., 2016). Previous studies have shown that a prone position creates a physiological preload ranging from 140 to 240 N in vivo, and the lumbar spine experiences a compressive force of 200 N while sleeping (Dreischarf et al., 2016; Kienle et al., 2016; Wilke et al., 1999; Zhao et al., 2005).

2.5 Benchtop device testing parameters

Distraction of vertebrae at the beginning of the TLIF procedure was mimicked by seating the nose of the IFD between the platens at a depth of 21.3 mm prior to beginning impaction which allowed the compression platens to apply a lateral compressive load of approximately 200 N of force to the IFD when fully inserted between the platens. Compression clamps were used to aid in the compression of the platens, and alignment bolts were used to ensure that the platens remained parallel to each other.

SAWBONES® polyurethane foam sheets (20 pounds per cubic foot (PCF) bone foam, SAWBONES®, Vashon Island, WA, United States) were placed on the interior of each compression platen to replicate the cancellous bone of vertebral bodies. The 3 mm thick sheets were cut into 50.8 mm tall x 25.4 mm wide rectangles to fit into designated slots on the inside of the compression platens, lying flush against the platens. The IFD remained in contact with the SAWBONES® material until full insertion. New SAWBONES® pieces were used for each IFD insertion. The IFDs were Ti-6Al-4V, 12 mm in height and 24 mm in width, provided by Medtronic, Inc. (Memphis, TN, United States).

Based on the results from the drop weight material selection testing, a one-pound aluminum drop weight was used for all subsequent experimental testing, and only the drop height was varied. Drop heights ranged from 60 cm to 120 cm in 10-cm increments, resulting in seven test groups which were compared to historical cadaveric data (Graham et al., 2023). A new interbody fusion device was used for each drop height group, and each device was fully inserted eight times from its designated drop height (n = 8 insertions for each device per drop height). Sample size was chosen to match prior TLIF impaction characterization work, based on low variances previously observed in benchtop testing (Graham et al., 2023). For each insertion, the impact waveform, number of strikes to full insertion, compressive load, and implant displacement were recorded. A total of 56 complete insertions were performed (7 groups x 8 insertions/group). A summary of all testing parameters is shown in Table 1.

TABLE 1. Benchtop impact testing parameters.

Drop height (cm)Lateral compressive loadSpring constantIFDDrop weightInsertions
60708090100110120200 N63.22 lb/in12 mm × 24 mmTi-6Al-4V1 lb6061 Aluminumn = 8

2.6 Cadaveric data

The cadaveric data used for comparison against benchtop data is the same as used in our previous work (Graham et al., 2023). Cadaveric data is from two separate cadavers with no statistical differences observed between impact measurements recorded. IFD insertion data was collected from spinal levels T10 to S1.

2.7 Statistical analyses

Data analyses methodology remains the same as in previous work (Graham et al., 2023). For benchtop-to-cadaver comparisons and drop weight material comparisons, data were analyzed via one-way ANOVA and Dunnett’s test, with a significance level of α = 0.05. Drop height groups were compared only to historical cadaveric data (Graham et al., 2023) and were not compared to other drop heights. These analyses were performed using R (R Core Team, 2020) and RStudio 4.0.2 (RStudio Team, 2020). GraphPad Prism 8.4.3 was utilized to create all plots (GraphPad Software, Inc., San Diego, CA, United States).

3 Results

The predicted CORs for steel 1018, CP zinc, and 6061 aluminum for all testing drop heights are shown in Table 2. The CORs were calculated using Equations 1, 2 and the respective material properties for each drop weight material (AISI 1018 Steel, cold drawn, 2023; ASM Material Data Sheet, 2023; Makeitfrom, 2023). Graphs depicting these values can be found in Supplementary Figures S1-3.

TABLE 2. Coefficient of restitution (COR) predicted for drop weight materials at all drop heights.

Drop height (cm)Predicted velocity (m/s)MaterialCORplasticCORelastic
603.4Steel 10180.2000.154
CP Zinc0.1200.092
6061 Aluminum0.3160.243
703.7Steel 10180.1960.151
CP Zinc0.1170.090
6061 Aluminum0.3090.238
804.0Steel 10180.1920.148
CP Zinc0.1150.088
6061 Aluminum0.3030.233
904.2Steel 10180.1900.146
CP Zinc0.1130.087
6061 Aluminum0.2990.230
1004.4Steel 10180.1880.144
CP Zinc0.1120.086
6061 Aluminum0.2960.228
1104.6Steel 10180.1860.143
CP Zinc0.1110.085
6061 Aluminum0.2930.225
1204.8Steel 10180.1840.141
CP Zinc0.1100.084
6061 Aluminum0.2900.223

The impact duration and strike count to full IFD insertion for each drop weight material were compared to cadaveric data in Figures 4A,B, respectively. As the aluminum weight effectively replicated the impact duration from cadaveric waveforms, and all three materials resulted in strike counts that were not statistically different to those of the cadaver group, aluminum drop weights were used for all subsequent benchtop device testing.

Impact duration **(A)** and strike count **(B)** of steel 1018, CP zinc, and 6061 aluminum drop weights from benchtop testing at a 60 cm drop height, compared to cadaveric data (Graham et al., 2023) (“ns” designates no significant difference from cadaveric data; α = 0.05).
Figure 4. Impact duration (A) and strike count (B) of steel 1018, CP zinc, and 6061 aluminum drop weights from benchtop testing at a 60 cm drop height, compared to cadaveric data (Graham et al., 2023) (“ns” designates no significant difference from cadaveric data; α = 0.05).

For all benchtop insertion tests, the compressive load on the IFD and displacement of the IFD were monitored and recorded to ensure consistent compression and full insertion. Compression is plotted as an average of the compressive force in the left and right platens. An example of this data for the 100 cm drop height is shown in Figure 5 with an average of 7.125 strikes per test. The compression versus strike count plot (Figure 5A) demonstrates that consistent compression of approximately 200 N was maintained throughout the insertion process. The strike count versus IFD displacement plot (Figure 5B) verifies that the IFD displaced consistently with each strike and reached full insertion at a displacement of 35 mm.

Strike count versus **(A)** compressive force and **(B)** IFD displacement for n = 8 insertions at the 100 cm drop height using the aluminum drop weight.
Figure 5. Strike count versus (A) compressive force and (B) IFD displacement for n = 8 insertions at the 100 cm drop height using the aluminum drop weight.

The compressive load was further examined by investigating the average, maximum, and minimum compressive loads experienced by the IFD during each drop height scenario. As shown in Supplementary Table S1, the compressive load remained relatively constant during insertions from all drop heights with minimal variance. Supplementary Table S2 displays how the average displacement of the IFD per strike from each drop height remained notably consistent.

The number of strikes to full insertion, with full insertion defined as 1.5 times the length of the implant, was investigated for all drop heights and compared to the cadaveric dataset (Graham et al., 2023) (Figure 6). Of the drop heights tested, only the 60 cm drop height demonstrated equivalency to the strike counts from cadaveric TLIF. As expected, the highest number of strikes resulted from the 60 cm drop height, with an average of 12.88 strikes to full insertion. Strike count decreased as drop height increased, with the lowest number of strikes from the 120 cm drop height, with an average of 5.75 strikes to full insertion.

Cadaveric strike counts to full insertion (Graham et al., 2023) compared to the strike count for each drop height using an aluminum drop weight on the benchtop device (“ns” indicates no significant difference from cadaveric data; α = 0.05).
Figure 6. Cadaveric strike counts to full insertion (Graham et al., 2023) compared to the strike count for each drop height using an aluminum drop weight on the benchtop device (“ns” indicates no significant difference from cadaveric data; α = 0.05).

Figure 7 depicts the impact duration for all drop heights in comparison to the cadaver results. The impact duration for the cadaver group was matched by all drop heights.

Impact duration for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).
Figure 7. Impact duration for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).

Figure 8 shows the area under the impulse (force-time) curve for each drop height in comparison to cadaver results. The drop heights of 110 and 120 cm effectively replicated the area under the impulse curve of the cadaver group, while the area under the impulse curve was lower for the lower drop height groups.

Area under the impulse curve for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).
Figure 8. Area under the impulse curve for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).

The peak force of the impact from the drop weight at the various heights is shown in Figure 9. The peak impact force from cadaveric data was accurately mimicked by three benchtop groups: 90 cm, 110 cm, and 120 cm, while the peak impact force was lower for the other drop height groups.

Peak impact force for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).
Figure 9. Peak impact force for benchtop groups compared to the cadaver group (Graham et al., 2023) (“ns” indicates no significant difference from cadaveric data; α = 0.05).

The initial impact slope of the force-time curve for all benchtop groups is shown in Figure 10. No benchtop groups replicated the initial impact slope of the waveform seen in cadaveric data. Rather, initial impact slope of the waveform for all benchtop groups was lower than that from cadaveric TLIF.

Initial impact slope of the waveform for each benchtop group was lower compared to the cadaver group (Graham et al., 2023).
Figure 10. Initial impact slope of the waveform for each benchtop group was lower compared to the cadaver group (Graham et al., 2023).

The average values for each waveform characteristic—impact duration, area under the impulse curve, peak impact force, initial impact slope, and strike count—from all drop heights and cadaveric TLIF testing are displayed in Table 3. Notably, for all waveform characteristics, the cadaveric data exhibited greater variability compared to the benchtop data.

TABLE 3. Average values and standard deviation for all drop heights and cadaveric data (Graham et al., 2023) for impact duration, area under the impulse curve, peak impact force, initial impact slope, and strike count, using the 12 × 24 mm interbody fusion device.

Drop height (cm)Sample sizeImpact duration (μs)Area under the impulse curve (kN*μs)Peak Impact force (kN)Initial impact slope (kN/μs)Strike count
608107.2 ± 1.3644.0 ± 5.38.511 ± 0.230.1269 ± 0.01712.86 ± 0.99
708105.6 ± 3.8638.9 ± 419.153 ± 0.460.2200 ± 0.05411.00 ± 1.2
808107.3 ± 1.4721.2 ± 159.683 ± 0.290.1382 ± 0.0398.875 ± 0.83
908106.1 ± 1.8756.4 ± 1911.64 ± 0.490.2146 ± 0.0358.000 ± 1.1
1008106.9 ± 4.0765.4 ± 1811.52 ± 0.540.2620 ± 0.0577.125 ± 0.64
1108111.1 ± 1.1841.7 ± 1511.67 ± 0.740.2300 ± 0.0417.250 ± 0.46
1208109.5 ± 5.2891.9 ± 1012.08 ± 0.160.2634 ± 0.0405.750 ± 0.46
Cadaver16109.7 ± 16833.8 ± 9812.89 ± 2.20.4023 ± 0.1115.56 ± 5.1

The percent difference between each drop height and the cadaver waveform characteristic values is shown in Table 4. This data accentuates the differences and similarities of the benchtop data compared to cadaveric data.

TABLE 4. Percent difference between each benchtop group and the cadaveric group for impact duration, area under the impulse curve, peak impact force, initial impact slope, and strike count.

Percent difference from cadaveric data
Drop height (cm)Impact durationArea under the impulse curvePeak impact forceInitial impact slopeStrike count
60−2.24%−22.77%−33.96%−68.46%−17.27%
70−3.74%−23.37%−28.98%−45.31%−29.32%
80−2.18%−13.50%−24.86%−65.64%−42.97%
90−3.26%−9.28%−9.67%−46.67%−48.59%
100−2.59%−8.20%−10.62%−34.87%−54.22%
110+1.26%+0.94%−9.44%−42.83%−53.41%
120−0.14%+6.97%−6.26%−34.53%−63.05%

Table 5 summarizes the results of the comparison of benchtop data to cadaveric data, highlighting the drop height groups which mimicked cadaveric data. Impact duration was replicated by all drop heights. The area under the impulse curve was replicated by the 110 cm and 120 cm drop heights. Peak force was replicated by the 90 cm, 110 cm, and 120 cm drop heights. Initial impact slope was not replicated by any of the drop heights. Strike count was replicated by the 60 cm drop height group.

TABLE 5. Summary of benchtop groups which replicated cadaveric waveform characteristics. Check mark indicates no significant difference compared to the cadaver group.

Aluminum drop weight MassDrop height (cm)12 × 24 mm device
Impact durationArea under the impulse curvePeak impact forceInitial impact slopeStrike count
1.00 lb [0.454 kg]60✓​​​✓
70✓​​​​
80✓​​​​
90✓​✓​​
100✓​​​​
110✓✓✓​​
120✓✓✓​​

4 Discussion

The objective of this study was to refine an established, custom benchtop device (Graham et al., 2023) to more effectively replicate impact waveform characteristics which occur during cadaveric TLIF procedures. Our results demonstrate the benchtop model reliably simulated cadaveric TLIF impaction, providing a standardized supplement to cadaveric models for expediting the establishment of use condition parameters in IFD design and enhancing the LIF procedure (Graham et al., 2023; Palepu et al., 2021; Peck et al., 2018).

Thoroughly replicating cadaveric data using a benchtop device for IFD insertion necessitates measurement of the each waveform characteristic investigated in this study (Lu and Yu, 2003). Herein, refinement of the benchtop device enabled replication of three out of four waveform characteristics: impact duration, area under the impulse curve, and peak impact force. Notably, this is the first benchtop device to accurately replicate these three waveform characteristics from cadaveric data. The benchtop device in Palepu et al. matched only impact duration (Palepu et al., 2021), and in our previous work, only two waveform characteristics, peak impact force and initial impact slope of the force-time curve, were successfully replicated by the benchtop device (Graham et al., 2023).

Surprisingly, peak impact force plateaued beginning at 90 cm of drop height, presumably the result of frictional forces impeding the velocity of the drop weight within the drop tower. Future work will include the incorporation of velocity sensors in the benchtop device to determine if velocity is still increasing with drop height or if frictional forces may indeed be reducing peak impact forces. Impact duration, area under the impulse curve, and peak impact force mimicked cadaveric data from the two highest drop heights: 110 cm and 120 cm. Strike counts mimicked cadaveric data from only the 60 cm drop height. This tradeoff in drop heights and their ability to mimic waveform characteristics could suggest that matching all four waveform characteristics with a single drop height is unachievable. An increase in lateral compressive load would likely be required to prevent strike count from decreasing as drop height increases. Nonetheless, strike counts in the controlled benchtop model likely differ from strike counts in the more variable cadaveric environment due to the influence of many factors, including initial distraction of the disc space, oversizing of the implant relative to the spacer, use of different sized implants across spine levels, inconsistent force magnitude with mallet strikes, and angulation of the inserter during mallet strikes. In contrast, the benchtop testing distracted the platens to a distance consistent with the height of the IFD, did not utilize spacers, utilized only one IFD size, delivered consistent impact forces per insertion, and did not alter the angle of the inserter.

The modifications made to the benchtop device coupled with the changing of the drop weight material to a material with a higher COR were critical for replicating cadaveric data. The CORs in the experimental setup were expected to differ from the predicted CORs, as the drop weight materials impacted a stainless-steel button on the impact force sensor, while COR calculations were performed under the assumption of like materials impacting one another. Nonetheless, the drop weight material (i.e., COR) significantly affected the impact duration. Because aluminum possesses a higher COR than steel, the aluminum drop weight increased the impact duration, accurately mimicking cadaveric data for all drop heights. The higher COR of aluminum may have also contributed to the reduction in initial impact slope of the waveform compared to cadaveric data (Ahmad et al., 2016). It should be noted that this study was not designed to predict intra-operative performance. Furthermore, unlike the deliberate sizing of implants to vertebral spaces in cadaver testing, implant sizing relative to the platen spacing is not captured in benchtop testing. Instead, target lateral compressive forces are chosen to minimize variability between tests.

Assumptions regarding compressive load and spring constants were made in the implementation of the benchtop device. Compressive forces decreased by ∼3% as the IFD was inserted between the platens, presumably due to system compliance and possible minor misalignment of the platens and/or springs. Currently, there is no consensus on in vivo spinal compressive forces during the surgical procedure. Measurement of spinal compression and stiffness in cadavers could inform subsequent modifications to the device. While replication of the initial impact slope of the waveform was not the primary objective of this study, future work will include the exploration of methods to replicate the initial impact slope, as it represents the impact loading rate. Because the benchtop device can be easily modified, this work can also be expanded beyond the transforaminal approach to other LIF procedures such as lateral lumbar interbody fusion (LLIF). Additional limitations include the absence of variation in the coefficient of friction, which may result from variation in discectomy procedures and endplate prep by surgeons and could influence TLIF implantation mechanics. This represents a valuable direction for future research, as incorporating variable friction coefficients into experimental or computational models could further improve the translational impact of the work. Alternate methods of validation may also be pursued through finite element simulations and surgical procedures, in addition to cadaveric data. Future directions may also include testing different intervertebral fusion device materials, such as PEEK, to further strengthen the translational relevance of this research.

This research successfully validated the use of a benchtop impaction device for accurate replication of the energy absorption that occurs during cadaveric TLIF procedures. The energy absorption during IFD insertion was characterized by four aspects of the impact waveform: impact duration, area under the impulse curve, peak impact force, and initial impact slope of the waveform. Impact duration was replicated by all drop heights, area under the impulse curve was replicated by 110 cm and 120 cm drop heights, and peak impact force was replicated by 90 cm, 110 cm, and 120 cm drop heights, while cadaveric strike counts were replicated by the 60 cm drop height group. Overall, the ability to replicate the TLIF procedure in a benchtop device by mimicking in situ cadaveric impaction data will inform IFD design and use conditions, as well failure analyses methods, to ultimately refine LIF surgical techniques.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmede.2026.1548013/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Luca Ambrosio, Campus Bio-Medico University, Italy

Reviewed by: Lin Han, The Third Affiliated Hospital of Naval Medical University, China

Ajay Krishnan, Stavya Spine Hospital & Research Institute, India

Statements

Conflict of interest

Authors JR and MH were employed by Medtronic Spine and Biologics.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was supported by Medtronic, Inc., Memphis, TN [CW2258184] and the National Institutes of Health [R25GM123920]. L. Priddy was supported in part by the National Institutes of Health [P20GM103646]. Medtronic, Inc. was not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

CL: Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft, Writing – review and editing. MF: Investigation, Methodology, Writing – original draft. AG: Investigation, Methodology, Writing – original draft. TJ: Investigation, Software, Visualization, Writing – original draft, Writing – review and editing. HF: Investigation, Writing – review and editing. A-MJ: Investigation, Methodology, Writing – original draft. CH: Data curation, Software, Visualization, Writing – review and editing. JR: Conceptualization, Investigation, Methodology, Resources, Writing – original draft, Writing – review and editing. MH: Conceptualization, Investigation, Methodology, Resources, Writing – original draft, Writing – review and editing. LP: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing. MP: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review and editing.

Acknowledgements

The authors would like to thank Daniel Woods, Brian Murrell, Rex Armstrong, Chloe Chung, and Keith Miller from Medtronic, Inc. (Memphis, TN, United States) for technical input and guidance.

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Citation

Luke, C., et al. (2026). Impact energy during cadaveric transforaminal lumbar interbody fusion (TLIF) is replicated by a modular benchtop device. Frontiers in Medical Engineering, Volume 4. https://doi.org/
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4 - 2026

Copyright

Copyright © 2026 Luke, Foster, Graham, Jones, Faulkner, Janus, Hensley, Redmond, Henson, Priddy and Priddy.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Matthew W. Priddy

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