Numerical Investigation of Frictional Locking Mechanisms and Load Sharing in Pedicle Screw-Rod Assemblies Under Flexion: A Mechanically Consistent Finite Element Analysis

Öz

This study investigates the biomechanical stability and load-sharing mechanics of a pedicle screw-rod assembly under flexion using non-linear finite element analysis. A two-step analysis methodology was employed. First, a preload state equivalent to a 10 Nm tightening torque was approximated via an interference fit method using a non-linear Ti6Al4V model, and interface behavior was assessed through power screw theory and literature. Second, the developed model, which demonstrated mechanical consistency with experimental literature, was subjected to flexion loads utilizing non-linear material properties and contacts. Model 2.2 predicted a maximum moment capacity of 22.08 Nm, showing strong agreement with literature. Exceeding the maximum flexion force of 869.5 N triggered a significant internal load shift, where axial force on the screw head decreased while the reaction force on the tulip saddles nearly doubled, initiating macro-slip. Maximum von-Mises stresses exceeding the material's yield strength indicated localized yielding, facilitating a secure mechanical interlock. The model successfully captures the transition from static frictional locking to kinematic instability. By confirming the model's reliability through a comprehensive sensitivity analysis of solver parameters, this study demonstrates that the intrinsic frictional locking capacity, rather than solely geometry, is the primary determinant of system stability. Consequently, this research provides a robust and numerically consistent framework for evaluating the biomechanical performance of spinal implant designs.

Anahtar Kelimeler

• Pedicle Screw
• Finite Element Analysis
• Spinal Fixation
• Frictional Locking
• Pretension

Etik Beyan

The authors declare that no ethics approval was needed for this study.

Fleksiyon Altında Pedikül Vida-Rod Sabitlemelerinde Sürtünme Kilitleme Mekanizmalarının ve Yük Paylaşımının Sayısal İncelenmesi: Mekanik Olarak Tutarlı Sonlu Eleman Analizi

Öz

Bu çalışma, doğrusal olmayan sonlu elemanlar analizi kullanılarak fleksiyon altındaki bir pedikül vida-rod sisteminin biyomekanik stabilitesini ve yük paylaşım mekaniklerini araştırmaktadır. Öncelikle, 10 Nm'lik sıkma torkuna eşdeğer ön yük durumu, arayüzde deplasman kontrollü geçme durumu ile doğrusal olmayan Ti6Al4V modeli kullanılarak tahmin edilmiş, arayüz davranışı güç vidası teorisi ve literatür aracılığıyla değerlendirilmiştir. Ardından, deneysel verilerle mekanik tutarlılığı gösterilmiş olan model, doğrusal olmayan malzeme özellikleri ve temas koşulları kullanılarak fleksiyon yüklerine tabi tutulmuştur. Model 2.2, literatürdeki temel verilerle güçlü bir uyum sergileyerek 22,08 Nm’lik bir maksimum moment kapasitesi sağlamıştır. 869,5 N’luk en yüksek fleksiyon kuvvetinin aşılması, önemli bir iç yük kaymasını tetiklemiş ve vida başı üzerindeki eksenel kuvvet azalırken, tulip üzerindeki reaksiyon kuvveti neredeyse iki katına çıkarak makro kaymayı başlatmıştır. Malzemenin akma dayanımını aşan von-Mises gerilmeleri, bölgesel akmanın gerçekleştiğini ve güvenli bir mekanik kenetlenmeyi göstermiştir. Model, statik sürtünmeli kilitlenmeden kinematik kararsızlığa geçişi başarıyla yakalamaktadır. Çözücü parametreleri üzerinde yapılan kapsamlı bir duyarlılık analizi ile modelin güvenilirliği teyit edilerek, bu çalışmada sistem stabilitesini belirleyen temel faktörün yalnızca geometri değil, montajın sürtünmeli kilitlenme kapasitesi olduğu gösterilmiştir. Sonuç olarak bu araştırma, spinal implant tasarımlarının biyomekanik performansını değerlendirmek için sağlam ve sayısal açıdan tutarlı bir çerçeve sunmaktadır.

Anahtar Kelimeler

• Pedikül Vida
• Sonlu Elemanlar Analizi
• Omurga Sabitlemesi
• Sürtünmeli Kilitleme
• Ön Gerilme

Etik Beyan

Yazar bu çalışma için herhangi bir etik raporunun gerekmediğini bildirir.

Kaynakça

1. [1] Walker, C.T., Kakarla, U.K., Chang, S.W., and Sonntag, V.K.H. 2019. History and advances in spinal neurosurgery. J. Neurosurg. Spine, 31 (6), 775–785.
2. [2] Boos, N., and Webb, J.K. 1997. Pedicle screw fixation in spinal disorders: A European view. European Spine Journal, 6 (1), 2-18.
3. [3] Zdeblick, T.A. 1993. A prospective, randomized study of lumbar fusion. Preliminary results. Spine (Phila. Pa. 1976)., 18 (8), 983-991.
4. [4] Katonis, P., Christoforakis, J., Aligizakis, A.C., Papadopoulos, C., Sapkas, G., and Hadjipavlou, A. 2003. Complications and problems related to pedicle screw fixation of the spine. Clin. Orthop. Relat. Res., 411, 86-94.
5. [5] Faraj, A.A., and Webb, J.K. 1997. Early complications of spinal pedicle screw. European Spine Journal, 6 (5), 324-326.
6. [6] Cho, W., Shimer, A.L., and Shen, F.H. 2011. Complications Associated with Posterior Lumbar Surgery. Semin. Spine Surg., 23 (2), 101-113.
7. [7] DeWald, C.J., and Stanley, T. 2006. Instrumentation-related complications of multilevel fusions for adult spinal deformity patients over age 65: surgical considerations and treatment options in patients with poor bone quality. Spine (Phila. Pa. 1976)., 31 (19), 144-151.
8. [8] Lonstein, J.E., Denis, F., Perra, J.H., Pinto, M.R., Smith, M.D., and Winter, R.B. 1999. Complications associated with pedicle screws. Journal of Bone and Joint Surgery, 81 (11), 1519-1528.

9. [9] McAfee, P.C., Weiland, D.J., and Carlow, J.J. 1991. Survivorship analysis of pedicle spinal instrumentation. Spine (Phila. Pa. 1976)., 16 (8), 422-427.
10. [10] Galbusera, F., Volkheimer, D., Reitmaier, S., Berger-Roscher, N., Kienle, A., and Wilke, H.J. 2015. Pedicle screw loosening: a clinically relevant complication? European Spine Journal, 24 (5), 1005-1016.
11. [11] Cho, W., Mason, J.R., Smith, J.S., Shimer, A.L., Wilson, A.S., Shaffrey, C.I., Shen, F.H., Novicoff, W.M., Fu, K.M.G., Heller, J.E., and Arlet, V. 2013. Failure of lumbopelvic fixation after long construct fusions in patients with adult spinal deformity: Clinical and radiographic risk factors. J. Neurosurg. Spine, 19 (4), 445-453.
12. [12] Fogel, G.R., Reitman, C.A., Liu, W., and Esses, S.I. 2003. Physical characteristics of polyaxial-headed pedicle screws and biomechanical comparison of load with their failure. Spine (Phila. Pa. 1976)., 28 (5), 470–473.
13. [13] ASTM. 2024. ASTM F1798-24 Test Method for Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants.
14. [14] ASTM. 2021. ASTM F1717-21 Test Methods for Spinal Implant Constructs in a Vertebrectomy Model.
15. [15] Wu, L.C., Hsieh, Y.Y., Tsuang, F.Y., Chiang, Y.F., and Chiang, C.J. 2022. Deviating from the Recommended Torque on Set Screws Can Reduce the Stability and Fatigue Life of Pedicle Screw Fixation Devices. Medicina (Lithuania), 58 (6),808.
16. [16] Ardura, F., Chenaux, D., Pascal-Moussellard, H., and Hessmann, M.H. 2021. Evaluation of the reduction, tightening and gripping performance of an innovative set screw technology for instrumented posterior lumbar fusion: A biomechanical study. Orthopaedics and Traumatology: Surgery and Research, 107 (7), 102918.
17. [17] Rohlmann, A., Bergmann, G., and Graichen, F. 1994. A spinal fixation device for in vivo load measurement. J. Biomech., 27 (7), 961-967.
18. [18] Cetin, E., Özkaya, M., Güler, Ü.Ö., Acaroglu, E., and Demir, T. 2015. Evaluation of the effect of fixation angle between polyaxial pedicle screw head and rod on the failure of screw-rod connection. Appl. Bionics Biomech., 150649.
19. [19] Nemade, A., Shikalgar, A., Sancheti, S., and Wadkar, S.P. 2021. Biomechanical analysis of spinal pedicle screws under static compression and tensile bending. Mater. Today Proc., 47, 4778–4785.
20. [20] Sengul, E., Ozmen, R., and Demir, T. 2022. The effects of pre-stressed rods contoured by different bending techniques on posterior instrumentation of L4-L5 lumbar spine segment: A finite element study. Proc. Inst. Mech. Eng. H, 236 (7), 960–972.
21. [21] Wu, J., Feng, Q., Yang, D., Xu, H., Wen, W., Xu, H., and Miao, J. 2023. Biomechanical evaluation of different sizes of 3D printed cage in lumbar interbody fusion-a finite element analysis. BMC Musculoskelet. Disord., 24 (1), 1–12.
22. [22] Kim, K.H., Bae, J., Yoo, KJ., Jeong, S., Lee, BJ. 2026. Evaluation of the porosity and structural stability of 3D-printed porous titanium pedicle screws using finite element analysis. Biomed. Eng. Lett., 16, 55-65.
23. [23] Zhao, C., Zhang, M., Li, P., Lei, L., Zhou, Q., Liu, L. 2026. Finite element analysis of the pedicle screw in lumbar transpedicular dynamic stabilization that penetrates the lateral cortex of the vertebral pedicle. BMC Musculoskelet Disord. 27, 315.
24. [24] Eberle, S., Gerber, C., Oldenburg, G. Von, Hungerer, S., and Augat, P. 2009. Type of hip fracture determines load share in intramedullary osteosynthesis. Clin. Orthop. Relat. Res., 467 (8), 1972–1980.
25. [25] Özkaya, M., and Demir, T. 2024. Evaluation of the mechanical behaviour of the expandable wedge locked nail fixation in retrograde use: A finite element study. Comput. Biol. Med., 174, 108419.
26. [26] Özkaya, M., Tunalı, S., Köksal, İ., and Demir, T. 2023. Mechanical comparison of standard interlocking, clawed, and expandable wedge locked nail fixations: An experimental and numerical study. Injury, 54 (2), 379–394.
27. [27] Özkaya, M. 2021. İntramedüller çivilerin biyomekanik özelliklerinin deneysel olarak ve sonlu elemanlar yöntemi ile belirlenmesi. TOBB Ekonomi ve Teknoloji Üniversitesi, Fen Bilimleri Enstitüsü, Doktora Tezi, 165s, Ankara.