Improved Biomechanical Performance of Tibial Spine Fracture Repair With Suture and Anchor Fixation in Pediatric Cadaveric Knees

Abstract

Background:

Prior studies in porcine and adult human bone suggest that suture fixation is superior to screw fixation of pediatric tibial spine fractures (TSFs). However, we have previously demonstrated that 2-suture repair was biomechanically comparable with 2-screw repair in human pediatric cadaveric knees.

Purpose:

To evaluate whether TSF fixation with sutures attached to anchors placed in stronger metadiaphyseal bone would produce biomechanically superior repair to 2-screw and 2-suture constructs.

Study Design:

Controlled laboratory study.

Methods:

Six pediatric cadaveric knees were acquired. We applied the same TSF creation protocol used in our previous study, then repaired the fractures by passing 2 No. 2 FiberWire sutures through the fracture fragment and the base of the anterior cruciate ligament, with sutures passed through bony tunnels and secured to two 2.8-mm anchors in the metadiaphyseal cortex. This construct of suture plus suture anchor (suture anchor group) underwent the same biomechanical loading protocol used in our prior study, in which pediatric knees were randomly assigned to either screw fixation (n = 6; fractures reduced with two 4.0-mm cannulated screws and washers) or suture fixation (n = 6; fractures repaired as in the suture anchor group except the sutures were tied across a metaphyseal bony bridge after their exit from the bony tunnels). All specimens were mounted in flexion and biomechanically tested with cyclic loading followed by a load-to-failure test. New data were statistically compared with the prior study’s results.

Results:

The suture anchor group had a median age of 9.00 years, while the screw and suture groups had identical median ages (8.50 years). All groups had an identical number of samples of each laterality. The ultimate failure load differed significantly across fixation methods (P = .006), primarily driven by higher ultimate failure loads in the suture anchor group (225.50 ± 46.46 N) when compared with the screw group (143.52 ± 41.97 N; P = .01) and suture group (135.35 ± 47.94 N; P = .009).

Conclusion:

TSF fixation with sutures tied to metadiaphyseal suture anchors provided significantly stronger repair than 2-suture and 2-screw constructs.

Clinical Relevance:

The suture anchor fixation method for TSF may offer enhanced stability and durability to reduce the risk of postoperative complications while improving functional patient outcomes.

Tibial spine fracture (TSF) occurs in approximately 3 per 100,000 pediatric patients per year, most frequently affecting children between the ages of 8 and 14 years.^1,9,17 They comprise 2% to 5% of pediatric knee injuries and occur in about 14% of anterior cruciate ligament (ACL) injuries.^18,20,30 Often, TSF repair involves surgical fixation by screw or high–tensile strength suture fixation, followed by rehabilitation programs that focus on early motion restoration, minimized ACL laxity, and stability for bone healing.

In efforts to determine the optimal repair for these injuries, many studies have evaluated the biomechanical properties of different TSF fixation techniques. ^†† Most of the published literature,^{5,8,10,19,26,29} as well as a recent systematic review comparing 1-screw to 1 high–tensile strength suture repair constructs in porcine or adult human bone, ³⁴ have suggested that suture fixation is superior to screw fixation. However, these studies have primarily been conducted in adult cadaveric and porcine specimens, which have biomechanical properties distinct from the pediatric bone in which these injuries tend to occur. In general, pediatric bone is less dense and tolerates lower stress before failure, which can influence the quality of different TSF fixation methods.^7,11,31

In a study published in 2023, ¹⁶ we hypothesized that outcomes after both 2-screw and 2-suture repairs would be equivalent in human pediatric specimens and found no significant difference in the biomechanical properties of these 2 fixation methods. In that study, the key failure mechanism of the suture fixation group was characterized by the sutures pulling through the metaphyseal bone bridge over which they were tied. Moreover, the fixation constructs we tested in human pediatric bone failed at much lower loads than those reported for similar constructs tested in porcine and adult cadaveric bone. Therefore, there remains a need to identify improved fixation methods for pediatric TSFs.

In the current study, we aimed to evaluate the properties of TSF fixation with sutures attached to anchors placed in the stronger metadiaphyseal cortex in human pediatric knees. We applied the same biomechanical loading protocol used in our prior study ¹⁶ and sought to compare the characteristics of this new fixation method to the previously recorded data for the 2-screw and 2-suture repairs. We hypothesized that suture anchor–based TSF fixation would provide biomechanically superior repair by remedying the key mode of failure in the 2-suture group from our prior study.

Methods

Study Population

Institutional review board approval was deemed unnecessary for this study, as the tissues had been generously donated for research purposes by Allosource and the donors’ families, no genetic information was used, and there was no contact with any of the donors’ family members. In our prior study, 12 age- and laterality-matched pediatric cadaveric knees were obtained. In the present study, 6 more pediatric cadaveric knees were acquired. Each specimen was shipped overnight at –20°C and thawed for 24 hours at room temperature before dissection, fracture fixation, and biomechanical testing. Frozen specimens were preserved in tissue fluid that accompanied the tissue at the time of harvest. In the previous study, specimens were randomly assigned via a matched-pairs design to either the 2-screw (n = 6) or 2-suture (n = 6) fixation. In the present study, all 6 knees were assigned to TSF repair with 2 sutures plus 2 suture anchors (suture anchor group).

Sample Preparation and Fracture Creation

All soft tissue connections between the femur and tibia except for the ACL were dissected away from the specimens. Circumferential marks were made 2 mm away from the ACL insertion (Figure 1A). Then, a 5 mm–wide, 10 mm–long osteotome angled at 45° to the tibial plateau was used to induce a standardized Meyers-McKeever type 3 TSF, with each cut started at a previously made circumferential mark (Figure 2A). Fracture shape was that of an inverted-pyramid and fracture depth was approximately 7 mm, in accordance with reports in the literature describing the common morphology of type 3 TSFs.^2,13 Images of fracture creation and repair for each fixation type are shown in Figures 1 to 3.

Figure 1.

Fracture creation and repair with screw construct. (A) Specimen with representative circumferential marks made at 2 mm away from the ACL insertion. (B) Screws placed in a convergent trajectory through the medial and lateral aspects of the fracture fragment. (C) Anteroposterior fluoroscopy of the construct. (D) Screw-repaired construct loaded onto the electromechanical load frame for biomechanical testing. ACL, anterior cruciate ligament.

Figure 2.

Fracture creation and repair with suture construct. (A) Elevation of the anterior portion of the fracture fragment with the standard osteotome. (B) Suture passing through the anterior and posterior thirds of the ACL. (C) Suture fixation and reduction of the fracture and ACL. (D) Suture-repaired construct loaded on the electromechanical load frame for biomechanical testing. ACL, anterior cruciate ligament.

Figure 3.

Fracture creation and repair with suture anchor construct. (A) Fluoroscopic measurement of the physeal distance, defined as the distance from the superior margin of the tibial spine to the superior margin of the physis, which was approximately 1 cm in the specimen shown. (B) Sutures passing through the ACL and the ACL-fragment complex en route to tibial bony tunnels. (C) Sutures exiting their respective tibial bony tunnels, the location of the 2 cortical suture anchors, and the knots securing the sutures to the suture anchors. (D) The typical mode of failure for suture anchor constructs: fracture of the ACL-fragment complex. ACL, anterior cruciate ligament.

Fixation Types

Screw Fixation (Prior Study ¹⁶ )

For knees assigned to screw fixation, each fracture was reduced with two 1.25-mm K-wires (DePuy Synthes) drilled into the lateral and medial borders of the ACL-fracture construct and fracture bed at 45° angles. Then, K-wires were overdrilled using a 2.7-mm drill to a depth of 40 mm. Next, two 35 mm–length, 4 mm–diameter partially threaded cannulated screws (DePuy Synthes) were placed over the K-wires and tightened to adequate purchase (Figure 1B). Appropriate convergent screw trajectory and depth were assessed via anteroposterior, lateral, and axial fluoroscopy (OrthoScan).

Suture Fixation (Prior Study ¹⁶ )

For knees assigned to suture fixation, a tibial ACL guide (Arthrex) and 2.4-mm drill-tip guide wire were used to drill 2 medial-entry bony tunnels spaced 1 cm apart and 1 cm distal to the articular surface of the medial tibial plateau, as described in previous studies.^2,21 Bony tunnels exited into both the medial and the lateral base of the manually reduced fracture fragment. Drill exit holes at the level of the ACL insertion into the fracture fragment were consistently 1 cm apart and straddled the anteromedial ACL bundle. Next, 2 No. 2 FiberWire sutures (Arthrex) were passed through the base of the ACL with a curved needle. One suture was passed through the anterior one-third of the ACL, the other through the posterior one-third (Figure 2B). The sutures were then pulled through the bony tunnels with a suture passer (Smith & Nephew) and secured over the 1-cm bony bridge with 5 alternating surgical knots (Figure 2C).

Suture Anchor Fixation (Present Study)

In the present study, the 6 new pediatric knees assigned to the suture anchor group were repaired in the same way as suture specimens, with the following exception: instead of tying sutures across a metaphyseal bony bridge following their exit of the medial and lateral bony tunnels, the sutures were secured with knots to two 2.8-mm suture anchors made of No. 2 MagnumWire suture (Smith & Nephew), also placed 1 cm away from the midline medially and laterally, at a distance of 3 physis lengths from the tibial plateau (Figure 3, A-C). Otherwise, specimens in the suture anchor group were placed through identical dissection, fracture, biomechanical testing, and data collection protocols to those used in the other groups.

Biomechanical Testing

Each specimen was potted in Bondo epoxy putty (3M) into custom-made testing constructs. Specimens were then mounted for biomechanical testing on an electromechanical load frame (model 5944; Instron) (Figures 1D and 2D) at approximately 30° of flexion with anterior loads in line with the load frame motion to simulate typical ACL loading conditions. All testing occurred at room temperature, and specimens were kept moist with normal saline throughout biomechanical testing. After loading, each testing construct was subjected to cyclic preconditioning, which consisted of 20 cycles of loading between 5 and 25 N at a rate of 60 cycles/minute. Next, a cyclic loading protocol was applied to each specimen. This included 500 cycles between 5 and 75 N at a crosshead speed of 100 mm/minute. Upon the completion of cyclic loading, samples recovered for 30 minutes. Finally, a load-to-failure protocol was conducted at a rate of 0.5 mm/second.

Data Recording and Statistical Analysis

For each biomechanical test, displacement and load were recorded at 20 Hz. Following testing, load-displacement curves were constructed for all cyclic and load to failure tests (Bluehill for Instron). The primary outcome was ultimate failure load (in N). Ultimate failure load was defined as the maximum value recorded on the load-displacement curve. Secondary outcomes were stiffness, the slope of the linear portion load-displacement curve during the load-to-failure test (in N/mm), as well as fixation elongation (in mm), defined as the change in test construct displacement between the 5th and 500th cycle at 75 N of load. Mode of failure was macroscopically documented.

An a priori power analysis conducted before our first study revealed that 4 samples per fixation method (screws and sutures) would be sufficient to achieve a power of 0.8. As previously mentioned, the prior study included 6 specimens per group, superseding the sample size needed for adequate powering. Given the scarcity of pediatric cadaveric knees and the powering of our prior study, we elected to test 6 knees in the suture anchor group in the present study so that each fixation group had an equivalent number of specimens. Independent-samples t tests compared biomechanical properties between pairs of repair methods, while a 1-way analysis of variance test was used to compare biomechanical properties across all fixation types. The log-rank test was used to determine differences in survival between groups during cyclic loading. A threshold of .05 was used to assess statistical significance (R Environment for Statistical Computing).

Results

The age range of the 18 included pediatric cadaveric knees (6 knees in each group) was 5 to 11 years. The screw group and suture group had identical mean ages (8.30 years) and median ages (8.50 years), while the suture anchor group had a mean and median age of 9.33 and 9.00 years, respectively. All groups had an identical number of samples of each laterality.

The ultimate failure load significantly differed across fixation methods (P = .006). This relationship was primarily driven by higher ultimate failure loads in the suture anchor group (225.50.00 ± 46.46 N) when compared with the screw group (143.52 ± 41.97 N; P = .01) and the suture group (135.35 ± 47.94 N; P = .009). The ultimate failure load did not significantly differ between the screw group and suture group (P = .76). The screw group had the greatest stiffness during load-to-failure testing (21.79 ± 10.58 N/mm) and the least elongation over the course of the cyclic loading protocol (5.02 ± 2.43 mm). By contrast, suture fixation was the least stiff during load-to-failure testing (13.83 ± 6.82 N/mm) and had the most elongation during cyclic loading (8.46 ± 3.99 mm). Suture anchor fixation had intermediate stiffness (18.09 ± 2.33 N/mm) and elongation (7.23 ± 2.68 mm). However, differences in elongation (P = 0.24) and stiffness (P = .21) were not statistically different across groups (Table 1 and Figure 4).

Table 1.

Comparison of Biomechanical Properties of Screw, Suture, and Suture Anchor Fixations ^a

Biomechanical Property	Screw (n = 6) b	Suture (n = 6) b	Suture Anchor (n = 6)	P
Ultimate failure load, N	143.52 ± 41.97	135.35 ± 43.17	225.50 ± 46.46	.006
Stiffness during load to failure, N/mm	21.79 ± 10.58	13.83 ± 6.82	18.09 ± 2.33	.21
Elongation over the course of cyclic loading, mm	5.02 ± 2.43	8.46 ± 3.99	7.23 ± 2.68	.24
Specimens that survived cyclic loading, n (%)	5 (83.3)	5 (83.3)	6 (100.0)	≥.99

^aData are presented as mean ± SD unless otherwise indicated. Boldface P value indicates statistically significant difference between groups (P < .05).

^bData for the screw and suture fixations are from Johnstone et al. ¹⁶

Figure 4.

Box-and-whisker plots of (A) ultimate failure load, (B) stiffness, and (C) elongation over the cyclic loading period, by fixation type. The solid black horizontal line in each box represents the median value for each plot, while the perimeter of each box represents the first and third quartiles. The black lines extending from the box represent the data range.

One screw construct and 1 suture construct did not survive the cyclic loading protocol; however, all suture anchor constructs survived. At ultimate failure load, every screw construct failed at the level of the intact tibial cancellous bone; no failure occurred through the ACL–fracture fragment complex. Four suture constructs failed by pulling through the cortical bony bridge, while 2 failed by pulling through the ACL–tibial spine avulsion fracture complex. Each suture anchor construct failed because of a fracture of the ACL–tibial spine avulsion fragment complex (Figure 3D).

Discussion

This study was a continuation our search, begun in our prior work, ¹⁶ for the best biomechanical construct for pediatric TSF repair, finding that fracture fixation with sutures tied to suture anchors placed into the meta-diaphyseal cortex was biomechanically superior to the 2-suture and the 2-screw constructs in pediatric human cadaveric tissue. While this was a time-zero controlled laboratory study, the biomechanical superiority of suture anchor group fixation may have immediate clinical relevance, as this fixation method may offer enhanced stability and durability to reduce the risk of postoperative complications while improving functional patient outcomes. Arthrofibrosis is recognized as a primary complication of these injuries, and enhanced fixation strength may support earlier postoperative motion that may reduce the risk of arthrofibrosis.^6,25 These findings may have implications for the contemporary clinical management of pediatric TSFs.

While prior research conducted in porcine and adult human bone has suggested that suture fixation is biomechanically stronger than screw fixation for TSF repair, our group’s recent study challenged this notion by demonstrating biomechanical equipoise between 2-suture and 2-screw constructs in human pediatric cadaveric knees. ¹⁶ The present study was born from the need to remedy a perceived flaw in our prior report’s suture fixation group, where sutures tied across a 1-cm metaphyseal bridge tended to pull and cut directly through the bone, causing mechanical failure of the construct. In response to our prior study’s results, the senior author within our research team (K.G.S.) employed the suture anchor group fixation technique to repair Meyers-McKeever type 2 and 3 TSFs in clinical practice. This choice arose from concerns about the potential for suture fixations to fail or injure the patients’ metaphyseal bony bridges during the postoperative recovery period. As a result of the author’s early clinical experience, as well as the notion that the use of anchors would obviate the need to tie sutures across a metaphyseal bridge adjacent to 2 recently drilled bony tunnels, the decision was made to evaluate the technique in the biomechanics laboratory. Both our study hypothesis and the senior author’s clinical experience were validated when the fixations of the suture anchor group displayed a significantly superior mean ultimate failure load (P = .006), and all–suture anchor constructs survived the cyclic loading protocol, compared with 1 failure each in the screw and suture constructs. The suture anchor group exhibited increased stiffness during load-to-failure testing and reduced elongation during cyclic loading relative to the suture group. This is noteworthy, as increased stiffness and decreased elongation are advantages of 2-screw repair. Therefore, suture anchor group repair may diminish some comparative superiority of screw repair over suture-based repairs. On average, suture anchor group fixation experienced less deformation over time and exhibited reduced responsiveness to force compared with suture fixation, although to a lesser extent than screw fixation. However, many TSFs have some degree of comminution, which reduces the feasibility of screw fixation. These observations may carry implications for the long-term durability of the repair.

Many prior studies in adult cadaveric and porcine bone have evaluated different options for TSF repair, often reporting ultimate failure loads 2 to 3 times higher than those observed for suture and screw constructs reported in our prior study. ^‡‡ The higher failure loads seen in other studies are likely a direct result of the distinct differences in the biomechanical properties of pediatric human bone, adult human bone, and porcine bone. Importantly, human pediatric bone has generally lower density and ultimate failure loads.^7,11,31 These biomechanical differences are exacerbated with suture repair, as a large fraction of the load during cyclic and load-to-failure testing is placed across a 1-cm metaphyseal bridge adjacent to 2 bony tunnels. However, the suture anchor group repair remedies this biomechanical disadvantage, allowing these fixations to withstand higher loads prior to failure. Notably, 2 other studies have evaluated the use of suture anchors for TSF repair.^2,19 However, Li et al ¹⁹ used human adult cadaveric knees, and Anderson et al ² used skeletally immature porcine knees. Moreover, their use of suture anchors was radically different from the technique employed in this study: While Li et al and Anderson et al placed intra-articular suture anchors to provide direct fragment compression and fixation, we used suture anchors as a modality to secure the sutures to extra-articular tibial metadiaphyseal cortex. Therefore, to our knowledge, the technique reported in this study has not been previously described or biomechanically evaluated. The biomechanical superiority of this technique over high–tensile strength suture repair, which has been consistently reported as the optimal method for TSF fixation in previous literature, suggests that this novel fixation method may be the best pediatric TSF repair strategy available.

Limitations

This study has multiple notable limitations, many of which have been described extensively in our prior report. ¹⁶ However, several limitations pertinent to this follow-up study should be emphasized. First, the suture anchor fixations that are the focus of the current study were prepared and biomechanically tested at a different time than the suture fixations and screw fixations that served as their comparison groups. These fixation groups were evaluated at different times because of the limited availability of pediatric cadaveric tissue. However, the source from which each specimen was obtained, the steps taken to prepare each specimen, and the laboratory conditions under which each specimen was tested were identical. Second, the biomechanical testing presented in this study was conducted at “time zero” after fracture fixation, such that the results presented cannot perfectly reflect clinical fixation, which can involve fixation loosening, tissue accommodation, and patient healing. Third, this study presented results related to the biomechanical loading of TSF constructs in the anteroposterior plane at 30° of knee flexion, but it did not evaluate these constructs in additional planes, nor did it evaluate the impact of rotational forces on the fixation. These forces are common in later rehabilitation of ACL injury and TSFs.^4,15,23,24 Fourth, this study only considered fixation with all–suture based anchors. It did not evaluate press-fit suture anchors or other fixation technologies, which might have produced different results. Fifth, to our knowledge, there are no studies that have reported on the biomechanical forces experienced by TSF constructs in the early postoperative period during weightbearing or range-of-motion exercises, nor have there been case reports of repair sutures pulling through a metaphyseal bone bridge and causing fixation failure. Therefore, it is impossible to conclude whether the increased fixation strength observed in the tested suture anchor constructs will result in fewer clinical fixation failures.

Last, the mean and median specimen age in the suture anchor group were slightly greater than those of the suture and screw fixation groups. This limitation was unavoidable: human pediatric cadaveric tissue in the age ranges where TSFs frequently occur is exceedingly rare and is only available through the generosity of donor families and the organizations that care for the tissue. After the completion of our prior study, where we matched specimens into 2 groups of 6 based on age and laterality, we were not able to acquire a new age-matched cohort of specimens. Given that slightly older specimens are more likely than not to have denser and stronger bone, it is possible that some of the additional strength demonstrated by the suture anchor repairs relative to the screw and suture repairs was due to differences in patient age rather than differences in fixation strength. However, the age differences between fixation types were minimal, and the between-group differences in ultimate failure load were large. As a result, we believe that suture anchor group fixation of TSFs would be superior to 2-screw and 2-suture fixation in the setting of age- and laterality-matched pairs.

Conclusion

In this study, we introduced a novel fixation method for pediatric type 3 TSFs that consists of securing fracture reduction sutures to suture anchors placed in the tibial metadiaphyseal cortex. This technique produced a significantly stronger repair compared with 2-suture and 2-screw fixation, which has been favored in previous studies. The study results suggest that constructs with suture anchor group fixation may provide the strongest repair for these injuries in human pediatric specimens, which may have implications for the contemporary clinical management of pediatric TSFs.