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Locking design affects the jamming of screws in locking plates
S61 Injury, Int. J. Care Injured 49S1 (2018) S61–S65 Volume 49 Supplement 1 June 2018 ISSN 0020-1383
Contents lists available at ScienceDirect
Injury Plating of Fractures: current treatments and complications Guest Editors: Peter Augat and Sune Larsson
j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / i n j u r y
Locking design affects the jamming of screws in locking plates Sabrina Sandriessera,c,*, Markus Ruppb, Markus Greinwalda, Christian Heissb, Peter Augata,c, Volker Altb a b c
Institute of Biomechanics, Trauma Centre Murnau, Murnau, Germany Department of Trauma, Hand and Reconstructive Surgery, University Hospital Giessen-Marburg, Campus Giessen, Germany Institute of Biomechanics, Paracelsus Medical University Salzburg, Salzburg, Austria
K E Y W O R D S
A B S T R A C T
Locking compression plate Locking screw Seizing Increased insertion torque Removal torque
The seizing of locking screws is a frequently encountered clinical problem during implant removal of locking compression plates (LCP) after completion of fracture healing. The aim of this study was to investigate the effect of two different locking mechanisms on the seizing of locking screws. Specifically, the removal torques before and after cyclic dynamic loading were assessed for screws inserted at the manufacturerrecommended torque or at an increased insertion torque. The seizing of 3.5-mm angular stable screws was assessed as a function of insertion torque for two different locking mechanisms (Thread & Conus and Thread Only). Locking screws (n=10 for each configuration) were inserted either according to the manufacturerrecommended torque or at an increased torque of 150% to simulate an over-insertion of the screw. Half of the screws were removed directly after insertion and the remaining half was removed after a dynamic load protocol of 100,000 cycles. The removal torques of locking screws exceeded the insertion torques for all tested conditions confirming the adequacy of the test setup in mimicking screw seizing in locked plating. Screw seizing was more pronounced for Thread Only design (+37%) compared to Thread & Conus design (+14%; P<0.0001). Cyclic loading of the locking construct consistently resulted in an increased seizing of the locking screws (P<0.0001). Clinical observations from patients treated with the Thread & Conus locking design confirm the biomechanical findings of reduction in seizing effect by using a Thread & Conus design. In conclusion, both over-tightening and cyclic loading are potential causes for screw seizing in locking plate implants. Both effects were found to be less pronounced in the Thread & Conus design as compared to the traditional Thread Only design. © 2018 Elsevier Ltd. All rights reserved.
Introduction Locking plates in which the screws are locked into the holes of the osteosynthesis plate are frequently used implants in a variety of bone fractures to achieve a stable osteosynthesis at the fracture site. In combination with angular stable locking screws a direct platebone contact is no longer needed [1]. Especially in osteoporotic bones this offers several advantages including a reduction in the potential risk of screw loosening [2]. Although locking plate systems have become a gold standard in the treatment of many fractures their removal can be quite laborious and challenging. Difficulties in removal of locking screw implants can be as frequent as 38% [3]. Three major factors have been identified which may cause these frequently encountered removal problems in locking plates [4]. One factor is the destruction of the recess of the screw head. This can be caused by overly forceful screw insertion and by worn or inaccurately placed screwdrivers. Also, the screw
Drs. Sandriesser and Rupp contributed equally. * Corresponding author at: Institute of Biomechanics, Trauma Centre Murnau, Prof. Kuentscher-Str. 8, Murnau, Germany E-mail address: [email protected] (S. Sandriesser). 0020-1383/© 2018 Elsevier Ltd. All rights reserved.
recess can be damaged by vigorous removal of ingrown bone tissue. A second factor is cross-threading of the screw head thread with the threads in the plate hole by off-axis insertion of the screw [5]. While this might result in jamming of the screw head into the plate it more importantly compromises the fixation stability of the construct and has to be avoided [6]. The third factor is the seizing of locking screws into the holes of the locking plate, which was encountered very early after the introduction of the locking mechanism [7]. This seizing of the screw head into the plate of the hole is sometimes incorrectly associated with cold welding, which actually is very unlikely to occur between anodized titanium surfaces. Terminology to describe this phenomenon is inconsistent, probably because of its rather phenomenological description. Terms used to describe the phenomenon of seized locking screws include welding, galling, fretting and jamming [8]. Although the exact mechanisms leading to the seizing of locking screws remain unclear, some factors which may increase the risk of seizing have been identified. Most importantly the insertion torque used for the screw insertion directly affects the amount of jamming of the screw head into the plate by increasing contact forces at the screw-plate-interface [6,8]. Consequently, the manufactures provide torque-controlled screwdrivers for their locking plate systems and recommend their utilization. It has been suggested that
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the more consistent use of torque controlled screwdrivers might be able to alleviate the seizing problem by avoiding jamming the screw threads into the threads of the plate hole [4]. Another factor affecting the extent of seizing is the period of time the implant was inserted. Intraoperative difficulties during screw removal appear to be increased with longer time periods from implant insertion to removal [8,9]. Finally, design, geometry and size of the locking screw head have demonstrated to affect the seizing of screws. Screws with a diameter of 3.5 mm appear to be most frequently affected by screw seizing [9–11]. Screw head and locking hole designs differ among different plate manufacturers and result in differences in stiffness and strength of the fixation construct and might as well affect seizing of the screws [12]. Moreover, alternative locking mechanisms have been suggested also to address the problem of locking screw seizing and locking plate removal [13,14]. The aim of this study was to investigate the effect of different locking mechanisms and of the applied insertion torque on the seizing of locking screws. Specifically, the removal torque of two different locking screw designs was assessed, either inserted at the manufacturer-recommended torque or at an increased insertion torque before and after cyclic dynamic loading.
Fig. 1. Locking screw mechanisms used for the biomechanical test. Thread & Conus design (left, SK-3525-40-2, LOQTEQ® 3.5, aap Implantate AG) and Thread Only design (right, No. 413.040 Synthes LCP, Johnson & Johnson).
Material and methods The aim of this study was: 1) to assess the effect of locking mechanism design on screw seizing by comparing a threaded screw head design with a conus and thread design; 2) to evaluate the effect of cyclic loading on the extent of screw seizing; and 3) to determine the effect of over-tightening the screws during insertion. Biomechanical test-setup Two different locking plate constructs were tested (Fig. 1). One in which the locking mechanism is based on interdigitizing threads in the plate and in the screw head (Thread Only: Synthes LCP, Johnson & Johnson, New Brunswick, NJ, US) and one with a combination of a press fit conus and a short thread (Thread & Conus: LOQTEQ® 3.5, aap Implantate AG, Berlin, Germany). In both cases self-tapping angular stable cortical screws with a diameter of 3.5 mm and a length of 40 mm were used (aap: SK-3525-40-2; Synthes: 413.040). For the Thread & Conus group, 4-hole plates (aap, PG-3555-04-2) were used and for the Thread Only group 9-hole plates (Synthes 423.591) were cut into halves to obtain 4-hole plates as well. The two inner holes of each 4-hole construct were used for testing. The outer two holes were used to mount the plate on an aluminum block. Supported by a guidance hole in the second aluminum block an exact insertion angle of 0° could be obtained. To avoid shear forces acting on the screw-plate-assembly, the whole construct was mounted on a cross table for screw insertion/removal and a rocker for the cyclic loading setup, respectively (Fig. 2). To determine the effect of screw seizing two test setups were custom made: one setup for the insertion/removal of the locking screws into the plate holes and one setup to cyclically bend the screws in the plate. Screws were removed either immediately after insertion or after 100,000 load cycles. Insertion and removal of the screws was performed on a servohydraulic testing machine (Instron 8874, Instron Ltd., High Wycombe, UK) equipped with a torque transducer (Model QWLC-8M, Honeywell international Inc., Morriston, NJ, US) with a measuring range of ±5 Nm and an accuracy of 0.1% (Fig. 2, left). Locking screws were inserted either according to the manufacturer-recommended torque (2 Nm for aap and 1.5 Nm for Synthes) or at an increased torque of 150% (3 Nm for aap and 2.25 Nm for Synthes) to simulate an over-insertion of the screw. For each configuration and setup 10 screws were tested. To ensure that the screw insertion was not limited by the rotation angle of the testing machine, 75% of the defined insertion
Fig. 2. Test-setup for screw insertion and removal (left) and for dynamic loading of the screw (right). The plate was mounted on an aluminum block (1) with a guidance hole in a second aluminum block (2) to obtain an insertion angle of 0°. For dynamic testing bicortical loading of the screw was simulated by an H-shaped load applicator (4) parallel to the plate. In both test-setups the whole construct was either mounted on a cross table (3) or a rocker (5) to eliminate shear forces.
torque was applied manually with a torque limiting screw driver. The remaining percentage was applied by the testing machine at an angular velocity of 0.25°/s and a constant axial load of 50 N, representing an axial compressive load during surgery. For the test group without cyclic loading of the screw-plate-assembly, half of the screws were removed immediately after insertion at an angular velocity of 1°/s. The remaining half was removed after a dynamic load protocol conducted on an electromechanical testing machine (Instron E3000, Instron Ltd., High Wycombe, UK). The screws were positioned in an H-shaped load applicator to simulate bicortical fixation (Fig. 2, right). Cyclic loading of the screw was realized by a sinusoidal bending load of ± 150 N load amplitude (equivalent to ± 2.85 Nm cantilever bending) applied parallel to the plate at 5 Hz for 100,000 cycles. Additionally, a rocker was mounted between the load cell and the H-shaped load applicator to avoid shear forces. After dynamic testing, the screws were removed at an angular velocity of 1°/s in the servohydraulic testing machine. To simulate in-situ conditions all tests were conducted in saline solution (0.9% NaCl). Data analysis For statistical analysis a univariate ANOVA was performed to assess the effects of locking design, insertion torque and cyclic loading and their interaction. Paired t-tests were applied for the
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comparison of the insertion to the removal torque within each test group (IBM SPSS Statistics 19, Chicago, IL, US). Results The removal torque was calculated in percent as mean ± standard deviation with reference to the insertion torque (Fig. 3). A positive value indicates a higher removal torque compared to the insertion torque. All tested configurations showed removal torques to be higher than insertion torques. The univariate ANOVA revealed that overall removal torque was larger for the Thread Only design (+37%) as compared to the Thread & Conus design (+14%; P<0.0001) and that cyclic loading increased the removal torque (P<0.0001) for both designs. The effect of over-tightening the screws during insertion was different for the two different thread designs. For the Thread & Conus design overtightening had no effect on removal torque (P=0.62). In contrast, for the Thread Only design over-tightening showed an immediate increase in removal torque (+36%; P=0.0002) which ceased during cyclic loading (P=0.06). Removal of the screws directly after insertion at the manufacturer-recommended torque showed a removal torque which was slightly larger than the insertion torque for the Thread & Conus design (+13%; P=0.011) and which was similar to the insertion torque for the Thread Only design (+2%; P=0.45) At the increased insertion torque the removal torque for the Thread & Conus design was similar to the insertion torque (+5%; P=0.42) while the removal torque for the Thread Only design was substantially increased (+38%; P=0.002). Cyclic loading resulted in seizing of the screw-plate interconnection reflected by an increase of the removal torques. The seizing for the Thread & Conus design resulted in removal torques of about 120% of the insertion torques, for both the recommended (+18%; P=0.06) as well as the increased (+21%; P=0.004) insertion torque. The seizing in the Thread Only design was substantial for both screws inserted with the recommended insertion torque (+68%; p=0.002) and screws with the increased torque (+40%; P=0.008). Comparing both tested locking plate constructs, the Thread Only design showed significantly lower removal torques for the recommended insertion torque (+13% vs +2%; P=0.02), while for the increased insertion torque the removal torque of the Thread Only design was substantially increased (+5% vs +38%; P=0.002). Cyclic loading showed a significant effect on the removal torque for the recommended insertion torque with significantly lower removal torques for the Thread & Conus design compared to the Thread Only design (+18% vs +68%; P=0.003). Clinical examples Case 1 After a stumbling fall while walking her dog at a sidewalk, a 59-year-old postmenopausal woman was presented by paramedics with suspected right tibial fracture. Clinical examination showed a hematoma, swelling and pain of the right proximal lateral tibia. X-rays showed a lateral split impression fracture (AO type 4.1 B3) (Fig. 4). After CT scan of the right knee, operative treatment was carried out by open reduction and internal fixation with an anatomical LOQTEQ® proximal lateral tibia plate 3.5/4.5 (aap Implantate AG, Berlin, Germany) using an anterolateral approach. The depression zone was additionally augmented with Calcibone® Inject 2.5 ml (Zimmer Biomet, Warsaw, IN, US) after anatomical reconstruction of the articular surface. Partial weight-bearing of 20 kg for 6 weeks was allowed. The fracture showed consolidation without any loss of reduction and without any hardware failure on x-rays 8 weeks after trauma. Fifteen months postoperatively, the patient presented with free range of motion of her operated right knee without clinical signs of instability and requested
Fig. 3. The seizing of the screws is plotted as the exceeding removal torque in percent (mean±SD) compared to the defined insertion torque. The results are given for both tested groups (Thread & Conus and Thread Only), the two different insertion torques (100% and 150% of recommended torque) and with and without dynamic loading.
Fig. 4. Radiological series of a lateral split depression tibial plateau fracture (AO 4.1 B3) in a 59-year-old female after a stumbling fall on a sidewalk treated with open reduction and internal fixation with an anatomical LOQTEQ® proximal lateral tibia plate 3.5 /4.5. A1/A2: Lateral tibia plateau shows depression of the articular surface in the ap-view (A1). B: Coronal CT sections show the size of the joint depression (B1), which is underestimated in conventional X-rays. A split of the posterior tibial plateau is detected (B2). 3D reconstruction CT shows the lateral split with the impression of the tibia plateau as well (B3). C1/C2: X-rays on post-op day 1 with restoration of the tibia articular surface and correct implant position. D1/D2: X-rays 2 months after surgery with stable consolidation of the fracture without loss of reduction or implant failure. E1/E2: X-rays 15 months directly after uneventful hardware removal.
hardware removal. Finally, uncomplicated implant removal was carried out without any seizing of the locking screws with a duration of 23 minutes as an outpatient procedure. Case 2 A 39-year-old woman presented with bilateral genu varum (Fig. 5). She had previously undergone a knee arthroscopy procedure, in which a grade 4 cartilage lesion was treated by microfracturing and a degenerative lesion of the intermediate part of the medial meniscus was resected. Persisting pain and an additional grade 2 to 3 cartilage lesion at the medial femoral condyle together with a persisting grade 4 cartilage damage at the medial tibial plateau required revision surgery. Autologous chondrocyte transplantation in combination with a high tibia osteotomy was proposed as a
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Fig. 5. X-rays of a 39-year-old female with medial knee pain due to a symptomatic fourth-grade tibial cartilage damage with genu varum. Overcorrection of leg axis was performed with a LOQTEQ® high tibia osteotomy (HTO) plate. A: Long leg view of the right leg shows the Mikulicz line medial of the intercondylar region (mLPFA 85.9°, mLDFA 89.8°, mMPTA 88.5°, mLDTA 87.9°). B: Postoperative long leg view control shows the desired lateralization of the Mikulicz line and the correction of the mMPTA to 92.3° with correct positioning of the plate. C1/C2: Postoperative X-rays show the biplanar valgization high tibial open wedge osteotomy. The open wedge is still visible directly postoperative (C1). Tibial slope is physiological (C2). D1/D2: 6 weeks follow-up after surgery, consolidation of the osteotomy is visible (D1, D2). E1/E2: 16 months after surgery osteosynthesis material was removed without any problems.
two-stage procedure. After harvesting cartilage during stage 1 and subsequent chondrocyte in vitro cultivation, implantation of the chondrocytes together with a valgisation biplanar high tibia osteotomy for correction of her genu varum (mLPFA 85.9°, mLDFA 89.8°, mMPTA 88.5°, mLDTA 87.9°) was conducted. Postoperative movement was restricted to ROM E/F 0-0-60° for 4 weeks with partial weight-bearing of 20 kg for 6 weeks. Bony consolidation of the osteotomy at the intended correction of the axis was visible on x-rays after 6 weeks without any hardware failure. Sixteen months after surgery, the patient complained about persisting medial knee pain during physical activity. A combined arthroscopic procedure with nanofracturing of a remaining 1 cm diameter fourth-grade medial tibial cartilage damage close to a 1 cm diameter regenerated cartilage was carried in combination with uneventful hardware removal. After 12 weeks, free range of motion and full weightbearing were achieved. Discussion The effect of seizing of locking screws, which is clinically associated with difficulties in screw removal, could be reproduced with the described biomechanical test setup. The removal torques of locking screws exceeded the insertion torques for all tested conditions which confirms the adequacy of the test setup in mimicking screw seizing in locked plating. While screw seizing was observed for both types of locking designs, the extent of seizing was clearly reduced for the Thread & Conus design compared to the Thread Only design. Cyclic loading of the locking construct consistently resulted in an increased seizing of the locking screws. Clinical observations from patients treated with the Thread & Conus
locking design confirm the biomechanical findings of reduction in seizing effect by using a Thread & Conus design. In this biomechanical study the effects of over-tightening and cyclic loading were assessed in two different designs of locking screw mechanisms. Over-tightening is a frequently suspected reason for the seizing of screws in locking constructs [6,8]. Our findings confirmed that with the conventional Thread Only design over-tightening resulted in an increase of about 40% in the removal torque compared to tightening the screw with the manufacturerrecommended torque. In contrast, with the Thread & Conus design over-tightening had no effect on seizing of the locking screws. Thus the locking mechanisms based on a Thread & Conus design appeared not to be sensitive to over-tightening the screw and has the potential to reduce clinical problems with hardware removal. By cyclical loading of the locking screw we simulated the in situ loading situation experienced by osteosynthesis constructs during the course of fracture healing. With 100,000 load cycles about one to two months of normal to low activity during daily living were simulated [15]. With cyclic loading the removal torque of the locking screws was consistently increased by about 20% for the Thread & Conus design and by 40–70% for the Thread Only design. This finding suggests that locking screws are seized into the plate hole during their use and that with prolonged insertion time the seizing can become more important than the effect of over-tightening. In accordance with previous literature the seizing appears to be amplified during the time period of osteosynthesis implantation by cyclic deformations occurring at the screw-plate-interface [9]. Again, the Thread & Conus design appeared to be less affected by the seizing effect as compared to the Thread Only design. This study has its strengths and limitations. The study was carefully designed to investigate the two major effects of over-tightening and cyclic loading on screw seizing in two different locking screw designs. As inherent with most biomechanical studies we were unable to accurately simulate the biological process of bone growth around the screw and plate and the potential ingrowth of tissue on the screw surface or into the screw plate interface. Nevertheless, our setup initiated the process of screw seizing which is generated by mechanical loading of the screw interface. The load scenario in our test setup simulated axial loading of the bone and resulted in a combination of bending, compression and tension within the screw head/plate interface. Although we grossly examined the thread surfaces after screw removal no attempt was made to microscopically inspect the surfaces for mechanical wear, deformation or other sign of mechanical usage. In conclusion, the findings from this biomechanical study determined both over-tightening and cyclic loading as potential causes for screw seizing in locking plate implants. Both effects were found to be less pronounced in the Thread & Conus design as compared to the traditional Thread Only design. Thus, the Thread & Conus appears to be less prone to seizing and might result clinically in fewer complications during plate removal. For the Thread Only design our findings strongly support the recommendation to use a torque limiter for the insertion of locking screws and to adhere to the manufacturer-recommended insertion torques to avoid difficulties in locking screw removal. Disclosure The institution of the authors has received financial support for the mechanical testing by aap Implantate AG, Berlin, Germany. PA and VA are members of the Research Committee of the OTCF. Acknowledgment The authors of this manuscript express their thanks to the Osteosynthesis and Trauma Care Foundation for the sponsorship of the publication of this Supplement in Injury.
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References [1] Egol KA, Kubiak EN, Fulkerson E, Kummer FJ, Koval KJ. Biomechanics of locked plates and screws. J Orthop Trauma 2004;18:488–93. [2] Haidukewych GJ, Ricci W. Locked plating in orthopaedic trauma: a clinical update. J Am Adac Orthop Surg 2008;16:347–55. [3] Suzuki T, Smith WR, Stahel PF, Morgan SJ, Baron AJ, Hak DJ. Technical problems and complications in the removal of the less invasive stabilization system. J Orthop Trauma.2010;24:369–73. [4] Ehlinger M, Adam P, Simon P, Bonnomet F. Technical difficulties in hardware removal in titanium compression plates with locking screws. Orthop Traumatol Surg Res 2009;95:373–6. [5] Cole PA, Zlowodzki M, Kregor PJ. Treatment of proximal tibia fractures using the less invasive stabilization system: surgical experience and early clinical results in 77 fractures. J Orthop Trauma 2004;18:528–35. [6] Gallagher B, Silva MJ, Ricci WM. Effect of off-axis screw insertion, insertion torque, and plate contouring on locked screw strength. J Orthop Trauma 2014;28:427–32. [7] Hamilton P, Doig S, Williamson O. Technical difficulty of metal removal after LISS plating. Injury 2004;35:626–8. [8] Van Nortwick SS, Yao J, Ladd AL. Titanium integration with bone, welding, and
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screw head destruction complicating hardware removal of the distal radius: report of 2 cases. J Hand Surg Am 2012;37:1388–92. Fujita K, Yasutake H, Horii T, Hashimoto N, Kabata T, Tsuchiya H. Difficulty in locking head screw removal. J Orthop Sci 2014;19:304–7. Maehara T, Moritani S, Ikuma H, Shinohara K, Yokoyama Y. Difficulties in removal of the titanium locking plate in Japan. Injury 2013;44:1122–6. Bae JH, Oh JK, Oh CW, Hur CR. Technical difficulties of removal of locking screw after locking compression plating. Arch Orthop Trauma Surg 2009;129:91–5. Varady PA, von Ruden C, Greinwald M, Hungerer S, Patzold R, Augat P. Biomechanical comparison of anatomical plating systems for comminuted distal humeral fractures. Int Orthop 2017;41:1709–14. Plecko M, Lagerpusch N, Andermatt D, Frigg R, Koch R, Sidler M, et al. The dynamisation of locking plate osteosynthesis by means of dynamic locking screws (DLS): an experimental study in sheep. Injury 2013;44:1346–57. Bottlang M, Lesser M, Koerber J, Doornink J, von Rechenberg B, Augat P, et al. Far cortical locking can improve healing of fractures stabilized with locking plates. J Bone Joint Surg Am 2010;92:1652–60. Morlock M, Schneider E, Bluhm A, Vollmer M, Bergmann G, Muller V, et al. Duration and frequency of every day activities in total hip patients. J Biomech 2001;34:873–81.
Contents lists available at ScienceDirect
Injury Plating of Fractures: current treatments and complications Guest Editors: Peter Augat and Sune Larsson
j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / i n j u r y
Locking design affects the jamming of screws in locking plates Sabrina Sandriessera,c,*, Markus Ruppb, Markus Greinwalda, Christian Heissb, Peter Augata,c, Volker Altb a b c
Institute of Biomechanics, Trauma Centre Murnau, Murnau, Germany Department of Trauma, Hand and Reconstructive Surgery, University Hospital Giessen-Marburg, Campus Giessen, Germany Institute of Biomechanics, Paracelsus Medical University Salzburg, Salzburg, Austria
K E Y W O R D S
A B S T R A C T
Locking compression plate Locking screw Seizing Increased insertion torque Removal torque
The seizing of locking screws is a frequently encountered clinical problem during implant removal of locking compression plates (LCP) after completion of fracture healing. The aim of this study was to investigate the effect of two different locking mechanisms on the seizing of locking screws. Specifically, the removal torques before and after cyclic dynamic loading were assessed for screws inserted at the manufacturerrecommended torque or at an increased insertion torque. The seizing of 3.5-mm angular stable screws was assessed as a function of insertion torque for two different locking mechanisms (Thread & Conus and Thread Only). Locking screws (n=10 for each configuration) were inserted either according to the manufacturerrecommended torque or at an increased torque of 150% to simulate an over-insertion of the screw. Half of the screws were removed directly after insertion and the remaining half was removed after a dynamic load protocol of 100,000 cycles. The removal torques of locking screws exceeded the insertion torques for all tested conditions confirming the adequacy of the test setup in mimicking screw seizing in locked plating. Screw seizing was more pronounced for Thread Only design (+37%) compared to Thread & Conus design (+14%; P<0.0001). Cyclic loading of the locking construct consistently resulted in an increased seizing of the locking screws (P<0.0001). Clinical observations from patients treated with the Thread & Conus locking design confirm the biomechanical findings of reduction in seizing effect by using a Thread & Conus design. In conclusion, both over-tightening and cyclic loading are potential causes for screw seizing in locking plate implants. Both effects were found to be less pronounced in the Thread & Conus design as compared to the traditional Thread Only design. © 2018 Elsevier Ltd. All rights reserved.
Introduction Locking plates in which the screws are locked into the holes of the osteosynthesis plate are frequently used implants in a variety of bone fractures to achieve a stable osteosynthesis at the fracture site. In combination with angular stable locking screws a direct platebone contact is no longer needed [1]. Especially in osteoporotic bones this offers several advantages including a reduction in the potential risk of screw loosening [2]. Although locking plate systems have become a gold standard in the treatment of many fractures their removal can be quite laborious and challenging. Difficulties in removal of locking screw implants can be as frequent as 38% [3]. Three major factors have been identified which may cause these frequently encountered removal problems in locking plates [4]. One factor is the destruction of the recess of the screw head. This can be caused by overly forceful screw insertion and by worn or inaccurately placed screwdrivers. Also, the screw
Drs. Sandriesser and Rupp contributed equally. * Corresponding author at: Institute of Biomechanics, Trauma Centre Murnau, Prof. Kuentscher-Str. 8, Murnau, Germany E-mail address: [email protected] (S. Sandriesser). 0020-1383/© 2018 Elsevier Ltd. All rights reserved.
recess can be damaged by vigorous removal of ingrown bone tissue. A second factor is cross-threading of the screw head thread with the threads in the plate hole by off-axis insertion of the screw [5]. While this might result in jamming of the screw head into the plate it more importantly compromises the fixation stability of the construct and has to be avoided [6]. The third factor is the seizing of locking screws into the holes of the locking plate, which was encountered very early after the introduction of the locking mechanism [7]. This seizing of the screw head into the plate of the hole is sometimes incorrectly associated with cold welding, which actually is very unlikely to occur between anodized titanium surfaces. Terminology to describe this phenomenon is inconsistent, probably because of its rather phenomenological description. Terms used to describe the phenomenon of seized locking screws include welding, galling, fretting and jamming [8]. Although the exact mechanisms leading to the seizing of locking screws remain unclear, some factors which may increase the risk of seizing have been identified. Most importantly the insertion torque used for the screw insertion directly affects the amount of jamming of the screw head into the plate by increasing contact forces at the screw-plate-interface [6,8]. Consequently, the manufactures provide torque-controlled screwdrivers for their locking plate systems and recommend their utilization. It has been suggested that
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the more consistent use of torque controlled screwdrivers might be able to alleviate the seizing problem by avoiding jamming the screw threads into the threads of the plate hole [4]. Another factor affecting the extent of seizing is the period of time the implant was inserted. Intraoperative difficulties during screw removal appear to be increased with longer time periods from implant insertion to removal [8,9]. Finally, design, geometry and size of the locking screw head have demonstrated to affect the seizing of screws. Screws with a diameter of 3.5 mm appear to be most frequently affected by screw seizing [9–11]. Screw head and locking hole designs differ among different plate manufacturers and result in differences in stiffness and strength of the fixation construct and might as well affect seizing of the screws [12]. Moreover, alternative locking mechanisms have been suggested also to address the problem of locking screw seizing and locking plate removal [13,14]. The aim of this study was to investigate the effect of different locking mechanisms and of the applied insertion torque on the seizing of locking screws. Specifically, the removal torque of two different locking screw designs was assessed, either inserted at the manufacturer-recommended torque or at an increased insertion torque before and after cyclic dynamic loading.
Fig. 1. Locking screw mechanisms used for the biomechanical test. Thread & Conus design (left, SK-3525-40-2, LOQTEQ® 3.5, aap Implantate AG) and Thread Only design (right, No. 413.040 Synthes LCP, Johnson & Johnson).
Material and methods The aim of this study was: 1) to assess the effect of locking mechanism design on screw seizing by comparing a threaded screw head design with a conus and thread design; 2) to evaluate the effect of cyclic loading on the extent of screw seizing; and 3) to determine the effect of over-tightening the screws during insertion. Biomechanical test-setup Two different locking plate constructs were tested (Fig. 1). One in which the locking mechanism is based on interdigitizing threads in the plate and in the screw head (Thread Only: Synthes LCP, Johnson & Johnson, New Brunswick, NJ, US) and one with a combination of a press fit conus and a short thread (Thread & Conus: LOQTEQ® 3.5, aap Implantate AG, Berlin, Germany). In both cases self-tapping angular stable cortical screws with a diameter of 3.5 mm and a length of 40 mm were used (aap: SK-3525-40-2; Synthes: 413.040). For the Thread & Conus group, 4-hole plates (aap, PG-3555-04-2) were used and for the Thread Only group 9-hole plates (Synthes 423.591) were cut into halves to obtain 4-hole plates as well. The two inner holes of each 4-hole construct were used for testing. The outer two holes were used to mount the plate on an aluminum block. Supported by a guidance hole in the second aluminum block an exact insertion angle of 0° could be obtained. To avoid shear forces acting on the screw-plate-assembly, the whole construct was mounted on a cross table for screw insertion/removal and a rocker for the cyclic loading setup, respectively (Fig. 2). To determine the effect of screw seizing two test setups were custom made: one setup for the insertion/removal of the locking screws into the plate holes and one setup to cyclically bend the screws in the plate. Screws were removed either immediately after insertion or after 100,000 load cycles. Insertion and removal of the screws was performed on a servohydraulic testing machine (Instron 8874, Instron Ltd., High Wycombe, UK) equipped with a torque transducer (Model QWLC-8M, Honeywell international Inc., Morriston, NJ, US) with a measuring range of ±5 Nm and an accuracy of 0.1% (Fig. 2, left). Locking screws were inserted either according to the manufacturer-recommended torque (2 Nm for aap and 1.5 Nm for Synthes) or at an increased torque of 150% (3 Nm for aap and 2.25 Nm for Synthes) to simulate an over-insertion of the screw. For each configuration and setup 10 screws were tested. To ensure that the screw insertion was not limited by the rotation angle of the testing machine, 75% of the defined insertion
Fig. 2. Test-setup for screw insertion and removal (left) and for dynamic loading of the screw (right). The plate was mounted on an aluminum block (1) with a guidance hole in a second aluminum block (2) to obtain an insertion angle of 0°. For dynamic testing bicortical loading of the screw was simulated by an H-shaped load applicator (4) parallel to the plate. In both test-setups the whole construct was either mounted on a cross table (3) or a rocker (5) to eliminate shear forces.
torque was applied manually with a torque limiting screw driver. The remaining percentage was applied by the testing machine at an angular velocity of 0.25°/s and a constant axial load of 50 N, representing an axial compressive load during surgery. For the test group without cyclic loading of the screw-plate-assembly, half of the screws were removed immediately after insertion at an angular velocity of 1°/s. The remaining half was removed after a dynamic load protocol conducted on an electromechanical testing machine (Instron E3000, Instron Ltd., High Wycombe, UK). The screws were positioned in an H-shaped load applicator to simulate bicortical fixation (Fig. 2, right). Cyclic loading of the screw was realized by a sinusoidal bending load of ± 150 N load amplitude (equivalent to ± 2.85 Nm cantilever bending) applied parallel to the plate at 5 Hz for 100,000 cycles. Additionally, a rocker was mounted between the load cell and the H-shaped load applicator to avoid shear forces. After dynamic testing, the screws were removed at an angular velocity of 1°/s in the servohydraulic testing machine. To simulate in-situ conditions all tests were conducted in saline solution (0.9% NaCl). Data analysis For statistical analysis a univariate ANOVA was performed to assess the effects of locking design, insertion torque and cyclic loading and their interaction. Paired t-tests were applied for the
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comparison of the insertion to the removal torque within each test group (IBM SPSS Statistics 19, Chicago, IL, US). Results The removal torque was calculated in percent as mean ± standard deviation with reference to the insertion torque (Fig. 3). A positive value indicates a higher removal torque compared to the insertion torque. All tested configurations showed removal torques to be higher than insertion torques. The univariate ANOVA revealed that overall removal torque was larger for the Thread Only design (+37%) as compared to the Thread & Conus design (+14%; P<0.0001) and that cyclic loading increased the removal torque (P<0.0001) for both designs. The effect of over-tightening the screws during insertion was different for the two different thread designs. For the Thread & Conus design overtightening had no effect on removal torque (P=0.62). In contrast, for the Thread Only design over-tightening showed an immediate increase in removal torque (+36%; P=0.0002) which ceased during cyclic loading (P=0.06). Removal of the screws directly after insertion at the manufacturer-recommended torque showed a removal torque which was slightly larger than the insertion torque for the Thread & Conus design (+13%; P=0.011) and which was similar to the insertion torque for the Thread Only design (+2%; P=0.45) At the increased insertion torque the removal torque for the Thread & Conus design was similar to the insertion torque (+5%; P=0.42) while the removal torque for the Thread Only design was substantially increased (+38%; P=0.002). Cyclic loading resulted in seizing of the screw-plate interconnection reflected by an increase of the removal torques. The seizing for the Thread & Conus design resulted in removal torques of about 120% of the insertion torques, for both the recommended (+18%; P=0.06) as well as the increased (+21%; P=0.004) insertion torque. The seizing in the Thread Only design was substantial for both screws inserted with the recommended insertion torque (+68%; p=0.002) and screws with the increased torque (+40%; P=0.008). Comparing both tested locking plate constructs, the Thread Only design showed significantly lower removal torques for the recommended insertion torque (+13% vs +2%; P=0.02), while for the increased insertion torque the removal torque of the Thread Only design was substantially increased (+5% vs +38%; P=0.002). Cyclic loading showed a significant effect on the removal torque for the recommended insertion torque with significantly lower removal torques for the Thread & Conus design compared to the Thread Only design (+18% vs +68%; P=0.003). Clinical examples Case 1 After a stumbling fall while walking her dog at a sidewalk, a 59-year-old postmenopausal woman was presented by paramedics with suspected right tibial fracture. Clinical examination showed a hematoma, swelling and pain of the right proximal lateral tibia. X-rays showed a lateral split impression fracture (AO type 4.1 B3) (Fig. 4). After CT scan of the right knee, operative treatment was carried out by open reduction and internal fixation with an anatomical LOQTEQ® proximal lateral tibia plate 3.5/4.5 (aap Implantate AG, Berlin, Germany) using an anterolateral approach. The depression zone was additionally augmented with Calcibone® Inject 2.5 ml (Zimmer Biomet, Warsaw, IN, US) after anatomical reconstruction of the articular surface. Partial weight-bearing of 20 kg for 6 weeks was allowed. The fracture showed consolidation without any loss of reduction and without any hardware failure on x-rays 8 weeks after trauma. Fifteen months postoperatively, the patient presented with free range of motion of her operated right knee without clinical signs of instability and requested
Fig. 3. The seizing of the screws is plotted as the exceeding removal torque in percent (mean±SD) compared to the defined insertion torque. The results are given for both tested groups (Thread & Conus and Thread Only), the two different insertion torques (100% and 150% of recommended torque) and with and without dynamic loading.
Fig. 4. Radiological series of a lateral split depression tibial plateau fracture (AO 4.1 B3) in a 59-year-old female after a stumbling fall on a sidewalk treated with open reduction and internal fixation with an anatomical LOQTEQ® proximal lateral tibia plate 3.5 /4.5. A1/A2: Lateral tibia plateau shows depression of the articular surface in the ap-view (A1). B: Coronal CT sections show the size of the joint depression (B1), which is underestimated in conventional X-rays. A split of the posterior tibial plateau is detected (B2). 3D reconstruction CT shows the lateral split with the impression of the tibia plateau as well (B3). C1/C2: X-rays on post-op day 1 with restoration of the tibia articular surface and correct implant position. D1/D2: X-rays 2 months after surgery with stable consolidation of the fracture without loss of reduction or implant failure. E1/E2: X-rays 15 months directly after uneventful hardware removal.
hardware removal. Finally, uncomplicated implant removal was carried out without any seizing of the locking screws with a duration of 23 minutes as an outpatient procedure. Case 2 A 39-year-old woman presented with bilateral genu varum (Fig. 5). She had previously undergone a knee arthroscopy procedure, in which a grade 4 cartilage lesion was treated by microfracturing and a degenerative lesion of the intermediate part of the medial meniscus was resected. Persisting pain and an additional grade 2 to 3 cartilage lesion at the medial femoral condyle together with a persisting grade 4 cartilage damage at the medial tibial plateau required revision surgery. Autologous chondrocyte transplantation in combination with a high tibia osteotomy was proposed as a
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Fig. 5. X-rays of a 39-year-old female with medial knee pain due to a symptomatic fourth-grade tibial cartilage damage with genu varum. Overcorrection of leg axis was performed with a LOQTEQ® high tibia osteotomy (HTO) plate. A: Long leg view of the right leg shows the Mikulicz line medial of the intercondylar region (mLPFA 85.9°, mLDFA 89.8°, mMPTA 88.5°, mLDTA 87.9°). B: Postoperative long leg view control shows the desired lateralization of the Mikulicz line and the correction of the mMPTA to 92.3° with correct positioning of the plate. C1/C2: Postoperative X-rays show the biplanar valgization high tibial open wedge osteotomy. The open wedge is still visible directly postoperative (C1). Tibial slope is physiological (C2). D1/D2: 6 weeks follow-up after surgery, consolidation of the osteotomy is visible (D1, D2). E1/E2: 16 months after surgery osteosynthesis material was removed without any problems.
two-stage procedure. After harvesting cartilage during stage 1 and subsequent chondrocyte in vitro cultivation, implantation of the chondrocytes together with a valgisation biplanar high tibia osteotomy for correction of her genu varum (mLPFA 85.9°, mLDFA 89.8°, mMPTA 88.5°, mLDTA 87.9°) was conducted. Postoperative movement was restricted to ROM E/F 0-0-60° for 4 weeks with partial weight-bearing of 20 kg for 6 weeks. Bony consolidation of the osteotomy at the intended correction of the axis was visible on x-rays after 6 weeks without any hardware failure. Sixteen months after surgery, the patient complained about persisting medial knee pain during physical activity. A combined arthroscopic procedure with nanofracturing of a remaining 1 cm diameter fourth-grade medial tibial cartilage damage close to a 1 cm diameter regenerated cartilage was carried in combination with uneventful hardware removal. After 12 weeks, free range of motion and full weightbearing were achieved. Discussion The effect of seizing of locking screws, which is clinically associated with difficulties in screw removal, could be reproduced with the described biomechanical test setup. The removal torques of locking screws exceeded the insertion torques for all tested conditions which confirms the adequacy of the test setup in mimicking screw seizing in locked plating. While screw seizing was observed for both types of locking designs, the extent of seizing was clearly reduced for the Thread & Conus design compared to the Thread Only design. Cyclic loading of the locking construct consistently resulted in an increased seizing of the locking screws. Clinical observations from patients treated with the Thread & Conus
locking design confirm the biomechanical findings of reduction in seizing effect by using a Thread & Conus design. In this biomechanical study the effects of over-tightening and cyclic loading were assessed in two different designs of locking screw mechanisms. Over-tightening is a frequently suspected reason for the seizing of screws in locking constructs [6,8]. Our findings confirmed that with the conventional Thread Only design over-tightening resulted in an increase of about 40% in the removal torque compared to tightening the screw with the manufacturerrecommended torque. In contrast, with the Thread & Conus design over-tightening had no effect on seizing of the locking screws. Thus the locking mechanisms based on a Thread & Conus design appeared not to be sensitive to over-tightening the screw and has the potential to reduce clinical problems with hardware removal. By cyclical loading of the locking screw we simulated the in situ loading situation experienced by osteosynthesis constructs during the course of fracture healing. With 100,000 load cycles about one to two months of normal to low activity during daily living were simulated [15]. With cyclic loading the removal torque of the locking screws was consistently increased by about 20% for the Thread & Conus design and by 40–70% for the Thread Only design. This finding suggests that locking screws are seized into the plate hole during their use and that with prolonged insertion time the seizing can become more important than the effect of over-tightening. In accordance with previous literature the seizing appears to be amplified during the time period of osteosynthesis implantation by cyclic deformations occurring at the screw-plate-interface [9]. Again, the Thread & Conus design appeared to be less affected by the seizing effect as compared to the Thread Only design. This study has its strengths and limitations. The study was carefully designed to investigate the two major effects of over-tightening and cyclic loading on screw seizing in two different locking screw designs. As inherent with most biomechanical studies we were unable to accurately simulate the biological process of bone growth around the screw and plate and the potential ingrowth of tissue on the screw surface or into the screw plate interface. Nevertheless, our setup initiated the process of screw seizing which is generated by mechanical loading of the screw interface. The load scenario in our test setup simulated axial loading of the bone and resulted in a combination of bending, compression and tension within the screw head/plate interface. Although we grossly examined the thread surfaces after screw removal no attempt was made to microscopically inspect the surfaces for mechanical wear, deformation or other sign of mechanical usage. In conclusion, the findings from this biomechanical study determined both over-tightening and cyclic loading as potential causes for screw seizing in locking plate implants. Both effects were found to be less pronounced in the Thread & Conus design as compared to the traditional Thread Only design. Thus, the Thread & Conus appears to be less prone to seizing and might result clinically in fewer complications during plate removal. For the Thread Only design our findings strongly support the recommendation to use a torque limiter for the insertion of locking screws and to adhere to the manufacturer-recommended insertion torques to avoid difficulties in locking screw removal. Disclosure The institution of the authors has received financial support for the mechanical testing by aap Implantate AG, Berlin, Germany. PA and VA are members of the Research Committee of the OTCF. Acknowledgment The authors of this manuscript express their thanks to the Osteosynthesis and Trauma Care Foundation for the sponsorship of the publication of this Supplement in Injury.
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