1 Introduction

Anterior cruciate ligament (ACL) tears are one of the most commonly encountered orthopedic injuries with an annual incidence of 68.6 per 100,000 person-years.^1,2 Reconstruction of the ACL is known to have promising results, with postoperative knee stability achieved in up to 95 % of patients.^3,4 Traditionally, bone-patellar tendon-bone (BPTB) or hamstring autografts have been used, but these patients may encounter donor-site morbidity such as pain, weakness, loss of sensitivity, and tendon ruptures.^3,5–8 In contrast, allografts can often avoid these difficulties due to greater flexibility afforded to the surgeon and lack of donor-site morbidity. Achilles allografts, in particular allow greater manipulation of the bone plug, ease in salvage of the graft, fewer graft diameter variations, lower rupture rates than BPTB allografts, and increased strength with decreased failure.⁹

Despite the success rates in ACL reconstruction, complications requiring revision surgeries still occur and worsen clinical prognoses.² Inconsistencies in allograft preparation may also lead to future failure due to incorrect dimensions, excess bone removal, or bone graft fractures.⁴ Additionally, stability of the bone plug after being secured may also be dependent on height of the plug and the difference between the size of the bone plug and the tunnel. Variations in these factors may contribute to inconsistent patient results and higher failure rates.^10,11 Beyond graft failures, preparation of bone blocks may increase operative time, thereby predisposing patients to greater risk of complications associated with prolonged surgical duration.¹² Secondarily, the need for operating room efficiency is evident with growing costs estimated to be $16.21 per minute.¹³ Thus, weighing viable reconstruction options within allograft repair for standardized graft preparation to reduce operative time, incur improved healing, and minimize revision repair is an ongoing priority. Pre-shaped allografts have shown reduced time and effort for surgeons in glenoid reconstruction as well, with complications not exceeding beyond those seen with traditional graft preparation.¹⁴ Companies routinely prepare allografts for reduction of operating room time, costs, and complications.

The present study seeks to compare the biomechanical performance of pre-shaped Achilles allografts with calcaneus bone blocks by RTI Surgical with that of surgeon shaped calcaneus bone block Achilles allografts. Grafts were placed under cyclic loading followed by load-to-failure to compare differences and assess future clinical value. We hypothesized that there would be no difference in load-to-failure testing between pre-shaped bone graft allografts and surgeon-shaped bone block allografts.

2 Materials and methods

2.1

2.1 Graft preparation

The pre-shaped Achilles tendon with calcaneus is an allograft prepared by RTI Surgical with the goal of standardizing preparation and minimizing preparation time. Six Achilles tendons with pre-shaped 10 mm diameter calcaneus bone plugs (Group 1) (Fig. 1) were acquired from RTI Surgical, along with 6 unshaped allografts (Group 2) (Fig. 2). The pre-shaped bone plugs ranged from 2.25 cm to 3 cm in length. The unshaped allografts were all 2.5 cm in length. Prior to experimentation, grafts were stored in a freezer at −80 °C. Prior to manipulation in the experimental setting, the specimens were thawed while still in their RTI Surgical packaging, in warm water for 15 min. All samples were then evaluated for suitability for testing. Grafts from Group 2 were prepared by cutting the larger calcaneus block into smaller 10 × 25 mm bone plugs using a hand-held sagittal saw based on a technique paper by DeFroda et al.¹⁵ (Fig. 3). No modifications were made to samples from Group 1. Measurements of bone length, width, and thickness were subsequently taken with a micrometer for both groups.

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Fig. 1 An anterior (A) and lateral (B) view of the Achilles allograft with pre-shaped calcaneal bone graft.

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Fig. 2 An anterior (A) and lateral (B) view of the Achilles allograft with non-shaped bone calcaneal block prior to surgeon shaping.

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Fig. 3 An anterior (A) and lateral (B) view of Achilles allograft after surgeon-shaping of the calcaneal bone graft.

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2.3

2.3 Biomechanical testing

The calcaneal graft of the allograft was inserted into the artificial sawbone block and secured with a 7 × 20mm interference screw with the aid of 1.5 mm nitinol wire to ensure that the screw trajectory was parallel with the bone tunnel. The interference screw was advanced until it was flush with the allograft bone graft. The sawbone base was secured in the base fixture of an Instron ElectroPuls E3000 Linear-Torsion All-Electric Dynamic Test Instrument (Instron Corporation, Norwood, MA) and the tendon was attached to the actuator with a frozen clamp device (Fig. 4). All grafts were oriented to apply loads parallel to the axis of the bone tunnel. The loading process began with a 20 cycle pre-conditioning phase of 10N–50N uniaxial cyclic loading at a frequency of 0.1 Hz, followed by 500 cycles of 10N–150N at 0.5 Hz to simulate flexion and extension. Grafts that successfully completed cyclic loading were then tested to failure at a loading rate of 20 mm/min. Displacement and load was recorded electronically throughout all testing. In all cases, the point of failure was noted to be the maximum load prior to the first drop in load as seen on load-to-failure graphs. Data from specimens were analyzed via t-test with statistical significance set at a p value of <0.05. Mode of failure such as bone graft or tendon failure was determined via visual inspection.

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Fig. 4 The sawbone block is secured to the base of the Instron and the allograft tendon is secured with a frozen clamp device.

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3 Results

3.1

3.1 Types of failure for pre-shaped and surgeon-shaped bone graft

Of the 6 pre-shaped bone grafts, 3 (50 %) experienced a failure at the sawbone/screw interface, 2 (33 %) experienced a bone graft fracture, and 1 (17 %) experienced a tendon avulsion. The bone graft fractures occurred 1.5 cm from the distal end of the bone graft and 1 cm from the proximal end of the bone graft. The one which failed by tendon avulsion failed during cyclic loading at the 481th cycle and thus was not tested in load-to-failure. This avulsion failure specimen was the only pre-shaped bone graft that was 3.0 cm long; four others were 2.5 cm long, and one was 2.25 cm long. Following its failure, the screw was removed, and a bone fracture was also found 1 mm distal to the bone/tendon interface.

Of the 6 surgeon-shaped bone grafts, 3 (50 %) experienced a failure at the sawbone/screw interface (Fig. 5) and 3 (50 %) experienced a bone graft fracture (Fig. 6). For two of the grafts which experienced failure at the sawbone/screw interface, removal showed the bone graft was still intact. For one of the grafts which experienced a bone graft fracture, the fracture occurred 1.5 cm from the distal end of the bone graft. The other two grafts that experienced a bone graft fracture also failed via a concomitant tendonous avulsion mechanism (Fig. 6).

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Fig. 5 Image demonstrating failure at the sawbone/screw interface.

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Fig. 6 (A) Image demonstrating fracture of the allograft bone graft leading to failure. (B) Image demonstrating failure of the allograft due to bone graft fracture with tendon avulsion.

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3.2

3.2 Biomechanical properties measured during testing

Table 1 reports the biomechanical properties measured during cycling and load-to-failure testing. Point of failure was defined as the first peak in the load graph followed by any decline, regardless of subsequent increases. Examples for premade and surgeon-shaped graft failure points can be seen in Fig. 7a and b, respectively (Fig. 7). No significant differences between pre-shaped grafts and surgeon shaped grafts were detected (Table 1).

Table 1 Load-to-failure measurements (mean ± standard deviation) of displacement after cycling, stiffness, load at failure, and displacement at failure.

	Pre-shaped (n = 5)	Surgeon Shaped (n = 6)	p-value
Displacement after cycling (mm)	0.34 ± 0.07	0.45 ± 0.21	0.28
Stiffness between 100N and 250N (N/mm)	153.8 ± 15.2	155.9 ± 13.5	0.81
Failure load (N)	630.0 ± 178.4	525.7 ± 115.6	0.27
Displacement at failure load (mm)	5.69 ± 1.52	5.02 ± 1.27	0.45

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Fig. 7 Typical load-to-failure graph for pull-out strength testing.

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Table 2 reports the biomechanical properties for each individual test.

Table 2 Individual biomechanical data for each test. ∗Test 5 failed during cyclic loading due to tension avulsion on the 481th cycle.

Test #	Construct	Displacement after cycling (mm)	Stiffness between 100N and 250N (N/mm)	Failure load (N)	Displacement at failure load (mm)
1	Pre-Shaped	0.38	174.04	6.13	817.99
2	Pre-Shaped	0.25	151.69	6.53	764.67
3	Pre-Shaped	0.030	163.12	4.81	530.57
4	Pre-Shaped	0.37	144.66	7.53	676.11
5	Pre-Shaped	∗∗∗	∗∗∗	∗∗∗	∗∗∗
6	Pre-Shaped	0.42	135.39	6.55	427.95
7	Surgeon-Shaped	0.30	154.94	3.04	304.47
8	Surgeon-Shaped	0.34	162.99	6.74	566.72
9	Surgeon-Shaped	0.24	168.12	3.86	516.87
10	Surgeon-Shaped	0.74	146.75	6.26	565.87
11	Surgeon-Shaped	0.70	168.36	5.78	624.45
12	Surgeon-Shaped	0.040	134.22	6.91	619.63

4 Discussion

This study investigated the use of pre-shaped versus surgeon-shaped Achilles allografts with calcaneal bone blocks for ACL reconstruction. No significant difference was found in displacement after cycling, stiffness during load-to-failure testing, load at failure, and displacement at failure. The most common failure point for pre-shaped grafts was the sawbone/screw interface. Surgeon shaped grafts saw equal rates of failure at the sawbone/screw interface and at the site of bone graft.

One pre-shaped graft in our mechanical testing experienced a tendinous avulsion failure during the cyclical loading phase. This failure might be due to either poor graft tensile composition, potentially caused by variations in graft preparation technique or potentially due to the handling during experimentation.

Pre-shaped grafts seemed to demonstrate greater load and displacement at failure during load-to-failure testing, although this difference was not proven significant. This study is the first to mechanically evaluate pre-shaped versus surgeon-shaped bone grafts for ACL reconstruction. These findings suggest that there may be a potential benefit of pre-shaped grafts in ACL reconstruction, warranting further investigation into their impact on surgical efficiency. Pre-shaped grafts are designed to reduce operative time for ACL repair¹⁸. Prior research has shown increased operative time during ACL reconstruction to be associated with a higher risk of deep vein thrombosis, surgical site infections, sepsis, extended hospital stays, and readmissions.¹²

This study has several limitations. The sample size of 12 is relatively small, potentially limiting the generalizability of the results. While a larger sample size is desirable to strengthen the statistical power and increase confidence in the findings, it is important to consider the practical and ethical limitations of conducting large-sample extensive mechanical testing with the use of donor allografts. A post-hoc estimate suggested that to achieve 80 % power for significance would require a sample size of at least 68, which would be a difficult number of allograft specimen to accrue. One pre-shaped graft failed during the cycling stage, further reducing the number of data points in that group. This failure could be due to inherent properties of the specific graft or a result of the pre-shaping process, but the limited sample size makes it difficult to definitively determine the cause. This study focused solely on the biomechanical outcomes of the grafts. It did not evaluate longitudinal outcomes, such as long-term durability or potential tissue integration. Additionally, relevant factors such as operation time and surgeon learning curve were not considered. Future patient outcome studies with larger sample sizes and longer follow-up periods would be necessary to investigate these aspects comprehensively.

The only specimen to fail during cyclic loading was the longer 3 cm pre-contoured bone plug. Previous research has suggested that the length of surgeon-shaped bone plugs may have an influence on the ability of the allograft and interference screw construct to resist failure ¹⁹. It may be of worth for future studies to investigate if various sized pre-shaped bone plugs have improved load-to-failure, though this will require an increased sample size. Additionally, to gain a more complete understanding of the efficacy of pre-shaped bone grafts, future research should extend beyond biomechanical testing. Long-term studies are warranted to evaluate graft integration, potential for tissue rejection, and overall functional recovery in patients. These studies could incorporate patient-reported outcome measures to assess pain levels, range of motion, and functional limitations over time. Considering surgeon experience variables in future studies could be insightful. Comparing outcomes between junior and experienced surgeons would help determine if the use of pre-shaped grafts reduces the learning curve associated with the surgical procedure. This could have significant implications for improving patient outcomes and potentially reducing operative times.

5 Conclusion

In the setting of ACL allografts, this study investigated the difference in load-to-failure of pre-shaped and surgeon-shaped Achilles tendon allografts with calcaneal bone blocks. Displacement, stiffness, load at failure and displacement at failure were metrics of interest to assess fidelity of the allograft for future use in clinical settings. Though differences were not found to be statistically significant in this study, mean load at failure for the pre-shaped grafts trended higher than surgeon-shaped graphs. We believe that continued investigation of pre-shaped allografts may hold clinical value for saving operating room time, training less experienced surgeons, and most importantly, conferring improved outcomes to patients.

CRediT authorship contribution statement

Carl Edge: Study design, Data acquisition, data Formal analysis, initial and final manuscript drafting. Brady Ernst: Study Conceptualization and design, data Formal analysis, final draft editing. Aadi Sharma: Data Formal analysis, manuscript drafting and editing. Kush Savsani: Data Formal analysis, manuscript drafting and editing. James Satalich: Study Conceptualization and design, data acquisition, data Formal analysis, final draft editing. Conor O'Neill: Study Conceptualization and design, data Formal analysis, manuscript revisions and final draft editing. John Owen: Study design, data Formal analysis, final manuscript editing. J Brett Goodloe: Study Conceptualization and design, data Formal analysis, final manuscript editing.

Informed consent

Not Applicable.

Guardian/patient's consent

Not required or applicable for current study.

Declaration of originality

This article has not been published elsewhere and it has not been simultaneously submitted for publication elsewhere. The original electronic files of drawings, photos and the article are retained by the author until the conclusion of the publication process. This statement also confirms that all tables and figures are original work and no permissions are required.

Institutional ethical committee approval

This data contains no patient information. This study has been performed in accordance with the ethical standards in the 1964 Declaration of Helsinki and has been carried out in accordance with relevant regulations of the US Health Insurance Portability and Accountability Act (HIPAA). IRB approval was not required.

Funding

Grafts in this study were donated by RTI Surgical. Implants and screws were provided by Smith & Nephew. No direct financial compensation was received by the authors from either company. The sponsors had no role in study design, data collection, analysis, interpretation, or manuscript preparation.