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Published in final edited form as: Clin Biomech (Bristol). 2025 Sep 4;130:106667. doi: 10.1016/j.clinbiomech.2025.106667

Physeal-sparing Anterior Cruciate Ligament Reconstruction in Early Adolescent Porcine Models Provides Better Anterior Stability but Similar Tissue Function Compared to Juvenile Porcine Models

Kaan Gurbuz a,b,Ψ, Yukun Zhang a,c,Ψ, Logan Opperman d, Jeffery T Spang e, Matthew B Fisher a,c,e,*
PMCID: PMC12958063  NIHMSID: NIHMS2149002  PMID: 40957368

Abstract

Background:

Pediatric anterior cruciate ligament injuries have increased over the past two decades. Surgical reconstruction is often necessary to restore joint stability and prevent secondary injuries. Physeal-sparing anterior cruciate ligament reconstruction is commonly used in prepubertal patients to minimize the risk of growth disturbances. However, this technique has also been used for adolescents. The effect of age on joint stability and tissue function following physeal-sparing reconstruction remains unclear. This study aimed to compare initial biomechanical outcomes of physeal-sparing reconstruction within juvenile and early adolescent porcine joints.

Methods:

Twelve porcine hindlimbs (n=6 per age group) were tested using a robotic testing system. Anterior-posterior loading, compression loading, and varus-valgus torque were applied to the tibia in the intact, anterior cruciate ligament transected, and anterior cruciate ligament reconstructed states at 40° and 60° of flexion. Soft tissue forces were calculated by the principle of superposition.

Findings:

Physeal-sparing reconstruction in early adolescent joints resulted in superior anterior stability under anterior tibial loading and compression compared to juvenile joints. Medial collateral ligament distraction and medial meniscus compression contributions showed smaller changes following reconstruction in the early adolescent group at 60° of flexion. Other joint kinematics and tissue forces were similar between groups.

Interpretation:

These findings suggest that skeletal maturity influences joint stability following physeal-sparing anterior cruciate ligament reconstruction, supporting its viability for both younger and early adolescent patients. This study provides biomechanical insights into surgical treatments for young patients and underscores the importance of using age-specific models to evaluate pediatric surgical techniques.

Keywords: Knee, ACL, Pediatrics, Physeal-sparing, Robotic testing

INTRODUCTION

The number of pediatric anterior cruciate ligament (ACL) injuries has been increasing over the last 20 years (Beck et al., 2017; Shaw and Finch, 2017; Weitz et al., 2020). For complete ACL injuries, surgical ACL reconstruction (ACLR) is the currently preferred approach to maintain the joint stability and reduce the risk of meniscal injuries (Anderson and Anderson, 2015; Dumont et al., 2012; Ellis et al., 2023). However, there remains debate about the issue of growth disturbances including angular deformity and limb-length discrepancy when drilling through the opened growth plates in children with significant growth remaining (Collins et al., 2016; Gamble et al., 2023; Gracia et al., 2019; Wong et al., 2019a).

There are different types of common pediatric ACLR techniques with different approaches for drilling into bone. Complete transphyseal technique include transphyseal bone tunnels for both the femur and tibia. This is similar to the approach used for adults, and is commonly employed for older adolescent patients (Anderson, 2003; Ellis et al., 2023; Pagliazzi et al., 2023). The partial transphyseal approach only drills through the tibial physis, with an epiphyseal tunnel for the femur (Chambers et al., 2019; Ellis et al., 2023; Gamble et al., 2023; Patel et al., 2018). Physeal-sparing technique makes no damage to the growth plates by creating the tunnels in the epiphysis for femur and tibia (Ellis et al., 2023; Kocher et al., 2006; Pierce et al., 2017; Wong et al., 2019b; Xerogeanes et al., 2012). Theoretically, physeal-sparing ACLR leads to lower risk of physeal injury and less growth disturbances since there is no crossing of the growth plates, which makes this technique more common for patients younger than 11 years old who have more years of skeletal growth (Ellis et al., 2023; Patel et al., 2018). However, it has been reported that this technique has also been used widely for patients between 11–13 years old (Knörr et al., 2025; Patel et al., 2018). A review study with patients aged between 12–19 years showed better restoration of normal knee laxity following physeal-sparing ACLR compared to complete transphyseal and partial transphyseal techniques (Pagliazzi et al., 2023), but comparisons were not made between ages.

Thus, it is unclear if age affects the outcome of physeal-sparing ACL reconstruction within skeletally immature knees. ACL morphology and function, as well as joint laxity and stiffness, change during skeletal growth (Cone et al., 2019a, 2022; Hosseinzadeh and Kiapour, 2021; Howe et al., 2022). Therefore, it is critical to assess pediatric ACLR at different stages of skeletal development. Due to the extremely limited availability of cadaveric human pediatric knees, the porcine model is commonly used in musculoskeletal research because of its similarity to human knees in age-specific biomechanics and anatomy, as well as the availability of joints at specific ages during skeletal growth (Cone et al., 2017; Kiapour et al., 2015; Proffen et al., 2012). The porcine ACL undergoes a gradual shift toward a more vertical alignment, and there is a sex-specific difference in cross-sectional area during adolescence (Cone et al., 2019a; Howe et al., 2022), both of which are observed in humans (Cone et al., 2022; Hosseinzadeh and Kiapour, 2021). The human and porcine ACL both have anteromedial and posterolateral bundles, which contribute to anterior and rotational joint stability (Boguszewski et al., 2011; Howe et al., 2022; Kiapour et al., 2015). Additionally, with the porcine model it was found that the relative contributions of the bundles to joint stability change with growth (Cone et al., 2019a; Howe et al., 2022).

The size and orientation of the porcine ACL as well as the biomechanical function of the porcine joints change similarly as human knees during skeletal growth (Cone et al., 2022, 2019a, 2019b; Hosseinzadeh and Kiapour, 2021; Howe et al., 2022).

Therefore, the objective of this study was to investigate and compare the initial joint stability and tissue function following physeal-sparing ACL reconstruction in the juvenile and early adolescent porcine models. To accomplish this objective, we evaluated joint biomechanics before and after physeal-sparing ACLR using a robotic testing system. We hypothesized that physeal-sparing reconstruction would show better biomechanical outcomes for juvenile joints, consistent with the higher clinical use of this technique in this age group.

METHODS

Specimen Preparation

A total of 12 deep digital flexor tendons and 12 hindlimbs were collected from female Yorkshire cross-breed pigs aged 1.5 months (juvenile) and 4.5 months (early adolescent) following humane euthanasia (n = 6 tendons and legs/age group) (Figure 1). The animals were bred at the North Carolina State University Swine Educational Unit, and the animal use was approved by North Carolina State University Institutional Animal Use and Care Committee. After collection, the tissues were wrapped in phosphate-buffered saline (PBS)-soaked gauze and stored at −20°C.

Figure 1.

Figure 1.

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Physeal-sparing ACL reconstruction was performed in both juvenile and early adolescent knee joints using an age-matched flexor tendon allograft at the matched age. A 6-DOF robotic testing system was utilized to perform biomechanical testing. Joint stability and soft tissue force contributions following ACL reconstruction were calculated and compared between different age groups.

Biomechanical Testing Procedure

A 6-degree-of-freedom robotic testing system (KR300 R2500, KRC4, Kuka) along with a universal force-moment sensor (Omega160 IP65, ATI Industrial Automation) was used for biomechanical testing (Figure 1) (Cone et al., 2019a, 2019b; Howe et al., 2022). The joints were prepared as previously described (Cone et al., 2019b; Howe et al., 2022). The flexor tendons and joints were then thawed at room temperature overnight prior to robotic testing.

Each joint was attached to the robotic testing system using custom clamps with the femur fixed to a bottom platform and the tibia attached to the end effector of the robot. The coordinate system was defined for each joint using a 3D point digitizer (G2X MicroScribe). During each test, a passive path was initially obtained by applying flexion-extension to the joint from 40° to 90° while minimizing the loads along the other five degrees of freedom (Howe et al., 2022). Anterior-posterior (AP) forces, compression, and varus-valgus (VV) torques were then applied to the tibia at 40° and 60° of flexion. Loads were scaled by age group based on previous studies (Cone et al., 2019a; Howe et al., 2022). For juvenile joints, the loading conditions were 20 N AP force, 60 N compression, and 1 N·m VV torque, while for early adolescent joints, loading conditions were 80 N AP force, 120 N compression, and 4 N·m VV torque. For each specimen, the intact joint was first subjected to the loading conditions. Kinematic under the loads were recorded, then repeated while recording forces for the intact joint and after capsule removal. The same loading conditions were then applied after ACL transection (ACLT) while recording the kinematics. The kinematics from intact and ACLT states were then repeated while recording forces. ACL reconstruction with physeal-sparing technique was then performed, and the same loading conditions were applied, with kinematics recorded and then repeated while recording forces. The recorded ACLR kinematics were repeated after removing the graft. Lastly, kinematics from the intact, ACLT, and ACLR states were repeated with forces recorded after the sequential removal of the medial collateral ligament (MCL), lateral collateral ligament (LCL), medial meniscus (MMEN), lateral meniscus (LMEN), medial femoral condyle (MCON), and lateral femoral condyle (LCON).

Surgical Procedure

All reconstructions were performed by a single surgeon. Flexor tendons harvested from juvenile and early adolescent pigs were used as ACL grafts for corresponding age-matched joints. The diameter of the femoral and tibial tunnels was determined as 6 mm for juvenile joints and 8 mm for early adolescent joints, based on prior data on porcine ACL size across skeletal growth (Howe et al., 2022). The physeal-sparing technique utilized epiphyseal tunnels for both femur and tibia (Figure 1). A guide wire was inserted between the insertion of anteromedial (AM) and posterolateral (PL) bundle footprints on the tibia using an ACL tip aimer system at 55° (Smith & Nephew), followed by overreaming with a cannulated reamer (6 mm or 8 mm, Arthrex). Similarly, the femoral tunnel was achieved by inserting a guide pin between the center of the AM and PL bundles, followed by overdrilling. Femoral fixation was performed by a #2 FiberWire (Arthrex) with an ABS button (Arthrex), followed by 5 cycles of passive flexion-extension with 22 N pretension for preconditioning. Tibial fixation was then accomplished by an 11 mm × 20 mm staple (Arthrex) with 100 N pretension applied to the graft at maximum posterior translation at 40° of knee flexion.

Data Analysis

For each flexion angle, change in anterior-posterior tibial translation (Δ APTT) under applied anterior-posterior force, change in anterior tibial translation (Δ ATT) under applied compression, and change in varus-valgus rotation (Δ VV rotation) under applied VV torques were obtained when applying loading conditions. The Δ APTT and Δ ATT were normalized to the anterior-posterior tibial width from previous data (Howe et al., 2022). The anterior force contribution taken by the native ACL and reconstructed ACL graft under maximum anterior translation, compression, varus rotation, and valgus rotation were calculated using the principle of superposition (Debski et al., 2017; Fujie et al., 1995; Howe et al., 2022), and normalized as a percentage of the total anterior force in the joint. The anterior force contribution within the MCL was calculated under maximum anterior translation and compression, and then normalized to the total anterior force in the joint. The distraction contribution taken by the collateral ligaments and the compression contribution taken by the menisci were calculated under maximum varus and valgus rotation and then normalized as a percentage of the total distraction or compression force. For all tissue forces, the change in force contribution was calculated relative to the intact state. The absolute kinematics and force values during each state were included in supplemental materials (Table S3 – 5).

Statistics

Statistical analysis was conducted using Prism (GraphPad). Outlier testing was performed using the ROUT method (Q=10%). Outliers in ACL anterior force contribution under varus valgus rotation, MCL distraction contribution, and lateral meniscus compression contribution under compression were identified and removed. A two-sample t-test was utilized to compare kinematics and force contributions following physeal-sparing ACLR between juvenile and early adolescent joints. Statistical significance was defined as P < .05. Individual data points along with means and 95% confidence intervals were shown in the figures. Mean differences, 95% confidence intervals of differences, and P values for all statistics are included as supplemental information (Table S1 – 2).

RESULTS

Joint stability following the physeal-sparing technique was compared between juvenile and early adolescent joints (Table 1). The average Δ APTT relative to intact joint under anterior-posterior drawer did not differ significantly between the two age groups at 40° and 60° of flexion (P > .05) (Figure 2A). However, physeal-sparing in early adolescent joints showed significantly smaller Δ APTT at 40° of flexion when normalized by anterior-posterior distance of tibia (−0.07, 95% CI [−0.13, −0.01], P = .02) (Figure 2B). At 60° of flexion, the normalized Δ APTT slightly differed between the two age groups, but this difference was not statistically significant (−0.06, 95% CI [−0.13 to 0.004], P = .06) (Figure 2B). Under compression, physeal-sparing ACLR resulted in decreases of the average ATT in early adolescent joints (−1.7 mm at 40° and −1.6 mm at 60° relative to intact joint), which was 1.8 mm and 3.7 mm smaller than the change following physeal-sparing ACLR in juvenile joints (P = .01 for both) (Figure 2C). The normalized data showed similar results, with normalized Δ ATT in early adolescent joints significantly lower than that in juvenile joints at both flexion angles (40°: 95% CI [−0.07, −0.01], P = .02, 60°: 95% CI [−0.2, −0.02], P = .02) (Figure 2D). The average changes in VV rotational stability were consistent between juvenile and early adolescent joints following physeal-sparing ACLR (P > .05 for both 40° and 60°) (Figure 2E).

Table 1.

Figure 2 Early adolescent joints vs. juvenile joints: Joint laxity following physeal-sparing ACLR. T-test.

Flexion Angle Mean Diff. 95.00% CI of diff. P Value
2A Delta APTT 40° −0.98 −3.03 to 1.07 0.31
60° 0.05 −2.49 to 2.59 0.97
2B Normalized delta APTT 40° −0.07 −0.13 to −0.01 0.02
60° −0.06 −0.13 to 0.004 0.06
2C Delta ATT under compression 40° −1.85 −3.12 to −0.57 0.01
60° −3.75 −6.37 to −1.12 0.01
2D Normalized delta ATT 40° −0.04 −0.07 to −0.01 0.02
60° −0.10 −0.18 to −0.02 0.02
2E Delta VV 40° −0.52 −3.23 to 2.19 0.68
60° −0.14 −2.69 to 2.41 0.91
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Figure 2.

Figure 2.

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Joint stability following physeal-sparing ACL reconstruction was compared between juvenile and early adolescent porcine joints at 40° and 60° of flexion. The increases in APTT relative to intact joint were similar in joints at both ages (A) but smaller in early adolescent joints at 40° of flexion when normalized by joint size (anterior-posterior distance of the tibia, B). The physeal-sparing technique could better restore anterior joint stability under compression in early adolescent porcine joints compared to juvenile porcine joints at 40° and 60° of flexion for (C) absolute and (D) normalized values. (E) The ability to restore varus-valgus rotational stability was similar between ages. Data points presented with mean and 95% CI. Statistical significance (p<0.05) between states indicated (*).

Differences in the anterior force contribution between the reconstructed ACL graft and the native ACL were evaluated and compared following physeal-sparing ACLR (Table S1). Under maximum anterior translation at 60° of flexion, the average reduction in anterior force contribution carried by the ACL graft was less pronounced in juvenile joints (−13%) compared to early adolescent joints (−24%) but not statistically significant (95% CI [−28.9, −6.9], P = .20) (Figure 3A). The average changes in anterior force contribution were similar between ages under compression (P > .05) (Figure 3B), varus rotation (P > .05) (Figure 3C), and valgus rotation (P > .05) (Figure 3D) at 40° and 60° of flexion.

Figure 3.

Figure 3.

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Changes in the anterior force contribution carried by ACL as a percentage of the total anterior force applied in the joint were similar between juvenile and early adolescent joint following physeal-sparing ACL reconstruction under (A) maximum anterior translation, (B) compression, (C) varus, and (D) valgus rotation at 40° and 60° of flexion. ⊗ represents the ACL anterior force direction pointing into page. Data points presented with mean and 95% CI.

Additionally, the function of tissues following the physeal-sparing technique for collateral ligaments and menisci was measured (Table S2). Under anterior translation, the average decrease in ACL anterior force contribution following physeal-sparing ACLR shifted to the MCL in both age groups with no significant differences in increases of MCL contribution (P > .05) (Figure 4A). The average changes in anterior contribution of MCL under compression were also similar between the age groups (Figure 4B). Under varus-valgus rotation, the average changes in distraction contribution in the lateral collateral ligaments (LCL) following ACLR were comparable between the groups (Figures 4C). The changes in distraction contribution in MCL were 7% smaller in the early adolescent joints compared to juvenile joints (95% CI [−12.6, −1.32], P = .02) (Figure 4D). Compression contribution in the medial meniscus showed no differences between juvenile and early adolescent joint at 40° of flexion (P > .05). However, it increased by 7% in early adolescent joints under varus rotation at 60°, which was significantly smaller than the 20% increase in juvenile joints (95% CI [−25.6, −0.8], P = .04) (Figure 4E). Decreases in compression contribution were observed in the lateral meniscus under valgus rotation, with no significant difference between the age groups (P > 0.05) (Figure 4F).

Figure 4.

Figure 4.

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Changes in force contribution taken by other soft tissues following physeal-sparing ACL reconstruction were similar between juvenile and early adolescent joints, while increases in compression contribution by medial meniscus were higher in juvenile joints under varus rotation at 60° of flexion. (A) Change in anterior force contribution taken by medial collateral ligament (MCL) under anterior translation. (B) Change in anterior force contribution taken by MCL under compression. (C) Change in distraction contribution taken by lateral collateral ligament (LCL) under varus rotation. (D) Change in distraction contribution taken by MCL under valgus rotation. (E) Change in compression contribution taken by medial meniscus (MMEN) under varus rotation. (F) Change in compression contribution taken by lateral meniscus (LMEN) under valgus rotation. Data points presented with mean and 95% CI. Statistical significance (p<0.05) between states indicated (*).

DISCUSSION

The aim of this study was to compare the biomechanical outcomes of the physeal-sparing ACL reconstruction technique between two different age groups. Physeal-sparing ACLR in early adolescent joints resulted in superior anterior stability under anterior tibial load and compression compared to juvenile joints, contradicting our hypothesis. The average change in distraction force in the MCL and the average increase in compression force on the medial meniscus was smaller in early adolescent joints compared to juvenile joints at 60°. Other joint kinematics and tissue function were similar between the two age groups following physeal-sparing ACLR.

Patient age is a major factor in pediatric ACLR technique selection due to concerns about potential growth disturbances (Pierce et al., 2017). Several clinical studies have reported angular deformity and leg-length discrepancy after complete transphyseal reconstruction (Pierce et al., 2017), which has been commonly used in male patients 14 years and older and female patients 13 years and older (Patel et al., 2018). A case series (age range: 10.1 – 13.8 years) with a mean follow-up of 31.5 months indicated a high rate of growth disturbance in patients with more than 5 years of growth remaining following partial transphyseal ACLR (Chambers et al., 2019). Theoretically, physeal-sparing technique should have the minimal impact on growth due to the all-epiphyseal drilling. However, despite the belief that complete transphyseal techniques are preferred in older patients and physeal-sparing techniques are more common in younger patients, previous clinical data does not fully align with this trend (Collins et al., 2016; Pagliazzi et al., 2023; Pierce et al., 2017). A review study with 39 patients (mean age: 13 years) reported that physeal-sparing reconstruction was performed in 25% of the cases of angular deformity and 47% of leg-length discrepancy (Collins et al., 2016). The incidence of growth disturbance between a complete transphyseal cohort (13.5 years, 491 operations with 6-year follow-up) and a physeal-sparing cohort (12 years, 162 operations with 5-year follow-up) was also found similar (Pierce et al., 2017). While our study did not assess growth disturbances, we found that joint stability and tissue function following physeal-sparing ACLR differed between early adolescent and juvenile joints. These biomechanical differences may suggest that for the physeal-sparing technique, skeletal maturity influences post-operative stability, with early adolescent joints exhibiting superior anterior stability compared to juvenile joints. This result was surprising given the prominence of physeal-sparing techniques in younger patients. This may be attributed to the ACL orientation, which becomes more vertical with age (Howe et al., 2022), providing greater opportunity for graft placement to positively impact stability since previous studies have shown better joint stability when using less vertical femoral tunnel positioning (Araujo et al., 2015; Loh et al., 2003). Additionally, more developed secondary stabilizers may further contribute to superior stability in older joints. These findings indicate that skeletal maturity also plays a crucial role in post-operative joint stability, which could be a factor when interpreting clinical findings related to growth disturbances. However, in-vivo studies are needed to correlate these findings on initial joint function with long-term outcomes.

Beyond the risk of growth disturbances, pediatric patients face higher rates of graft failure compared to adults, largely due to higher activity levels and difficulties in adhering to post-operative restrictions (Maletis et al., 2017). Additionally, large joint laxity has been shown to increase the risk of ACL injuries and graft failure (Lindanger et al., 2021; Ramesh et al., 2005; Rupp et al., 2001; Uhorchak et al., 2003). A cohort study of adult patients indicated that an anterior joint laxity between 3 mm and 5 mm post-ACLR increased the likelihood of revision surgery, which could lead to long-term joint instability and impact athletic careers. (Lindanger et al., 2021). In this study, the increase of APTT following reconstruction ranged from 3.3 mm to 5.2 mm in both age groups, which aligns with previous porcine studies using a complete transphyseal technique (3.7 mm – 9.6 mm at 30°) (Debandi et al., 2011; Iriuchishima et al., 2009). Although the primary function of the ACL was similar between the two age groups, the change in anterior tibial translation under compression was smaller in early adolescent joints, indicating superior anterior stability under compressive loads. This may be due to greater joint stiffness and larger anatomical size in early adolescent knees (Cone et al., 2019b; Howe et al., 2022). Notably, the increase in compression load on the medial meniscus under varus torques was smaller in early adolescent joints, suggesting that physeal-sparing ACLR in juvenile joints could lead to increased meniscal compression forces, which may potentially lead to secondary injury.

In this study, the use of the robotic testing system provided controlled and repeatable 6-degree-of-freedom loading and kinematic conditions. Unlike manual manipulation of simplified loading rigs which may introduce variability due to lack of repeatability or non-physiologic constraints on motion due to limited degrees of freedom (Livesay et al., 1997), the current robotic testing system improves the reproducibility of measurements and comparisons between states within the same joint. Another strength is the use of age-specific porcine models. While previous pediatric studies used adult joints (Kennedy et al., 2011), they may not accurately represent the mechanical behavior, such as the ligament properties, joint laxity, and joint stiffness. Additionally, joint morphology also changes with age, including elongation of the femoral condyles, which alters joint geometry and ligament orientation (Cone et al., 2022, 2019a; Little et al., 2023). The age-specific porcine joints used in this study more closely replicate the development stage of pediatric knees, therefore enhancing the translational relevance of our findings.

This study has a few limitations. First, while statistical differences were identified, a larger sample size may reveal additional differences in joint kinematics and tissue forces. For example, the variation in anterior force contribution under valgus rotation suggests a potential difference that did not reach statistical significance, which may become clearer with a larger sample size (Figure 3D). Second, inherent species-specific differences remain although the porcine model is similar to humans. For example, the full extension of porcine knee is more restricted (40° of flexion) than in humans. Third, joint translation along the anterior-posterior axis was normalized using tibial width data from previous studies or same-aged joints rather than specimen-specific measurements. Lastly, this study provides a time-zero biomechanical analysis and does not account for long-term biological factors such as graft remodeling or muscle activation. Future in-vivo studies assessing long-term functional outcomes in age-specific animal models or human patients would be beneficial.

CONCLUSION

In conclusion, this study evaluated the physeal-sparing ACLR technique in juvenile and early adolescent porcine models. While physeal-sparing reconstruction provided superior anterior stability in early adolescent joints, overall joint stability and tissue function were similar between age groups. These findings suggest that physeal-sparing techniques may be a viable option not only for younger patients but also for adolescents. This study underscores the importance of using age-specific models to evaluate pediatric ACL reconstruction techniques and contributes valuable biomechanical insights into surgical treatments for treating ACL injuries in young patients.

Supplementary Material

Suppplemental Tables

ACKNOWLEDGEMENTS

We would like to thank the NC State College of Veterinary Medicine and Laboratory Animal Resources for their contribution to this work. Funding for this study was provided by NIH (R01 AR071985). One of the co-first authors, KG, was supported by The Scientific and Technological Research Council of Türkiye (BAYG-2219; Grant # 1059B192200) as a visiting post doctorate researcher.

Footnotes

CONFLICT OF INTEREST STATEMENT

All authors declare that they have no financial and personal relationships with other people or organizations that could inappropriately influence their work.

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