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Research Article| Volume 11, SUPPLEMENT 1, S100-S104, February 2020

Kirschner wire prepared pilot holes improve screw pullout strength in synthetic osteoporotic-type bone

Published:October 01, 2019DOI:https://doi.org/10.1016/j.jcot.2019.08.015

      Abstract

      Objectives

      To compare the pullout strength and maximal insertional torque of pilot holes prepared with the traditional twist drill bit versus a smooth Kirschner wire.

      Methods

      Pilot holes were prepared using a drill press with either a 2.5 mm twist drill bit or a 2.5 mm smooth Kirschner wire into 2 distinct polyurethane foam densities representing severe and mild osteoporotic bone. 3.5 mm cortical and 4.0 mm cancellous screws were then inserted freehand into the prepared holes. All permutations of pilot hole type, screw size and foam density were tested for maximum pullout strength and maximum insertional torque.

      Results

      Kirschner wire prepared pilot holes resulted in significantly higher pullout load than drill bit holes in low density blocks (P < 0.001), but not in high density blocks (P = 0.232). There was no statistical difference (P > 0.05) for maximum insertional torque in the pilot hole preparation type.

      Conclusion

      In severely osteoporotic bone, Kirschner wire pilot hole preparation may improve screw pullout strength.

      1. Introduction

      The incidence of osteoporotic fractures is expected to increase to 3 million people by 2025.
      • Burge R.
      • Dawson-Hughes B.
      • Solomon D.H.
      • et al.
      Incidence and economic burden of osteoporosis-related fractures in the United States, 2005-2025.
      Bone-screw interface is a major factor of stable internal fixation of fractures
      • Cornell C.N.
      Internal fracture fixation in patients with osteoporosis.
      ; an important contributor to the strength of the bone-screw interface is bone mineral density. As bone density decreases due to aging or osteoporosis, obtaining adequate stability of fixation constructs becomes challenging.
      • Stromsoe K.
      • Kok W.L.
      • Hoiseth A.
      • et al.
      Holding power of the 4.5 mm AO/ASIF cortex screw in cortical bone in relation to bone mineral.
      The initial strength of screw fixation is assessed by both pullout strength and maximal insertional torque.
      • Ricci W.M.
      • Tornetta 3rd, P.
      • Petteys T.
      • et al.
      A comparison of screw insertion torque and pullout strength.
      • Cleek T.M.
      • Reynolds K.J.
      • Hearn T.C.
      Effect of screw torque level on cortical bone pullout strength.
      • Thakkar S.C.
      • Langdale E.R.
      • Mears S.C.
      • et al.
      Can the "turn-of-the-nut" method improve cortical screw fixation?.
      • Tankard S.E.
      • Mears S.C.
      • Marsland D.
      • et al.
      Does maximum torque mean optimal pullout strength of screws?.
      Effects of insertion technique, pilot hole size, and insertion depth on pullout strength have been previously studied.
      • Wu Z.
      • Nassar S.A.
      • Yang X.
      Pullout performance of self-tapping medical screws.
      • Schoenfeld A.J.
      • Battula S.
      • Sahai V.
      • et al.
      Pullout strength and load to failure properties of self-tapping cortical screws in synthetic and cadaveric environments representative of healthy and osteoporotic bone.
      • Battula S.
      • Schoenfeld A.J.
      • Sahai V.
      • et al.
      The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone.
      • Collinge C.
      • Hartigan B.
      • Lautenschlager E.P.
      Effects of surgical errors on small fragment screw fixation.
      • Steeves M.
      • Stone C.
      • Mogaard J.
      • et al.
      How pilot-hole size affects bone-screw pullout strength in human cadaveric cancellous bone.
      Current AO guidelines recommend pilot hole preparation with a sharp fluted drill bit of varying diameter depending on the size of the screw to be used.
      • Müller M.E.
      • Perren S.M.
      • Allgöwer M.
      • et al.
      Manual of Internal Fixation : Techniques Recommended by the AO-ASIF Group.
      In severely osteoporotic bone, removing bone may decrease screw holding power. This is magnified in areas of cancellous bone where bicortical fixation is not possible such as the distal fibula. In addition, the current body of literature supports that the elastic properties of bone are essential in the stability of the bone-screw interface.
      • Finlay J.B.
      • Harada I.
      • Bourne R.B.
      • et al.
      Analysis of the pull-out strength of screws and pegs used to secure tibial components following total knee arthroplasty.
      • Abshire B.B.
      • McLain R.F.
      • Valdevit A.
      • et al.
      Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out.
      • Shirazi-Adl A.
      • Dammak M.
      • Zukor D.J.
      Fixation pull-out response measurement of bone screws and porous-surfaced posts.
      This is the first study to examine the effect of drill hole preparation with kirschner wires in comparison to drill bits.
      We believe that compacting the trabecular bone, instead of cutting and removing it, during hole preparation in a osteoporotic cancellous bone model would result in greater stability at the bone-screw interface; furthermore, using a cancellous screw would also increase the strength significantly.
      The hypothesis of the study was that the pullout strength and maximal insertional torque would be higher when a screw is inserted in a hole prepared with a K-wire compared to a twist drill bit using a low density foam block bone model.

      2. Materials and methods

      Synthetic bone surrogate (Pacific Research Laboratories, Vashon, WA) with densities of 240 mg/cm3 and 80 mg/cm3were used to represent mild and severe osteoporotic cancellous bone, respectively.
      • Genant H.K.
      • Block J.E.
      • Steiger P.
      • et al.
      Quantitative computed tomography in assessment of osteoporosis.
      The polyurethane foam blocks are widely accepted as synthetic bone surrogates due to its uniformity, low cost, and reproducibility thereby eliminating specimen related variability
      • Seebeck J.
      • Goldhahn J.
      • Morlock M.M.
      • et al.
      Mechanical behavior of screws in normal and osteoporotic bone.
      The blocks had no surrogate for the cortical layer; these were used to attempt to isolate the effect of cancellous bone and its compaction to screw purchase and pullout strength. The large synthetic bone blocks were cut into 4 smaller blocks with a size of 3x18 × 4 cm. Each block was allocated to test 4 screws. The pilot holes were drilled with a drill press with either a 2.5 mm trochar tip smooth K-wire (Stryker Inc, Kalamazoo, MI) or a 2.5 mm twist drill bit (Synthes Inc, West Chester, PA). Although 2.5 mm K-wires are not standard in most orthopaedic instrument trays, this size was selected in order to minimize variables.
      Stainless steel 3.5 × 30 mm cortical and 4.0 × 30 mm cancellous screws (Synthes Inc, West Chester, PA) were inserted into the synthetic bone with two different levels of density.
      Maximal insertional torque was measured with a digital torque screwdriver (HTG2, Imada, Northbrook, IL). Screws were inserted manually with continuous motion as would be done clinically. The torque measurement was concluded when the screw stripped the threads in foam block.
      To test for pullout strength a different set of screws were inserted manually according to standard AO techniques to a depth of 25 mm to allow the pullout adapter to grip the screw under the screw head; then the screw was tightened to allow affect of compaction and compression of cancellous bone, as is seen clinically. Each block was secured into a universal materials testing machine (Instron ElectroPuls, Norwood, MA) using clamps. Each screw was attached to the load cell through an adapter which gripped the screw under the screw head (Fig. 1).
      Fig. 1
      Fig. 1Showing block secured with universal materials testing machine. Window in lower left shows close up of screw attached to load cell through adapter.
      The block was positioned such that the screw was in line with the testing machine crosshead, ie. pullout force action line. Screws were pulled out at constant rate of 0.1 mm/s until failure. The failure was defined as a rapid drop in force after the maximal pullout force point.
      The load-displacement data were collected. The pullout load was defined as the maximum load encountered during test before screw failure. Preliminary tests and power analysis showed that 80% power could be achieved using 12 samples with a 95% confidence. Thus, 12 trials were performed for each permutation of bone density, screw size and pilot hole preparation resulting in a total of 96 trials. Statistical significance was set at α = 0.05. The effects of the three main factors, namely, pilot hole preparation technique (K-wire and drill), density (high and low), and diameter (3.5 and 4.0 mm), as well as their interactions on the pullout and insertion torque were investigated via a three way ANOVA test. Furthermore, Pearson correlation analyses were conducted between insertional torque and load for each case of density and screw diameter.

      3. Results

      In severe osteoporotic bone (80 mg/cm3) the 3.5 mm cortical screw had a pullout strength of 78.3 ± 7.0 N (mean ± standard deviation) with a drill pilot hole versus 87.9 ± 6.1 N with a K-wire pilot hole. The 4.0 mm cancellous screw had a pullout strength of 94.8 ± 4.5 N with a drill pilot hole versus 107.0 ± 4.1 N with a K-wire pilot hole (Fig. 2).
      Fig. 2
      Fig. 2Pullout strength in 80 mg/ml bone surrogate.
      In the mild osteoporotic bone (240 mg/cm3) the 3.5 mm cortical screw had a pullout strength of 477.5 ± 41.4 N with a drill pilot hole versus 460.6 ± 41.0 N with a K-wire pilot hole. The 4.0 mm cancellous screw had a pullout strength of 580.8 ± 14.7 N with a drill pilot hole versus 576.0 ± 15.5 N with a K-wire pilot hole (Fig. 3).
      Fig. 3
      Fig. 3Pullout strength in 240 mg/ml bone surrogate.
      The three way ANOVA analysis showed that there was no effect of pilot hole preparation type on the pullout load (P = 0.996); However, there were significant effects of density (P < 0.001) and diameter (P < 0.001) factors. There was no interaction between the factors of pilot hole preparation technique and diameter (P = 0.418) in contrast with pilot hole preparation technique and density (P = 0.019) and diameter and density (P < 0.001). Thus, two separate two-way ANOVA tests were run for each density level in order to unveil the dichotomous effect of pilot hole preparation type and screw diameter on the pullout load. Test results showed that the K-wire provided significantly higher pullout load than drill bit in low density blocks (P < 0.001), but not in high density blocks (P = 0.232). While the screw diameter had a significant effect on the pullout load for each density level (P < 0.001), there was no significant interaction between the pilot hole preparation technique and screw diameter (P > 0.05).
      In the severe osteoporotic bone (80 mg/cm3) the 3.5 mm cortical screw had maximum insertional torque of 161 ± 16 Nmm with a drill pilot hole versus 174 ± 23 Nmm with a K-wire pilot hole. The 4.0 mm cancellous screw had maximum insertional torque of 206 ± 36 Nmm with a drill pilot hole versus 218 ± 40 Nmm with a K-wire pilot hole (Fig. 4).
      Fig. 4
      Fig. 4Maximal insertional torque in 80 mg/ml bone surrogate.
      In the mild osteoporotic bone (240 mg/cm3) the 3.5 mm cortical screw had maximal insertional torque of 1056 ± 158 Nmm with a drill pilot hole versus 1016 ± 82 Nmm with a K-wire pilot hole. The 4.0 mm cancellous screw had maximal insertional torque of 1253 ± 106 Nmm with a drill pilot hole versus 1260 ± 119 Nmm with a K-wire pilot hole (Fig. 5).
      Fig. 5
      Fig. 5Maximal insertional torque in 240 mg/ml bone surrogate.
      The three way ANOVA analysis showed that there was no effect of pilot hole preparation type on the insertional torque (P = 0.916); However, the effects of density (P < 0.001) and screw diameter (P < 0.001) were significant. There was no interaction between the factors of pilot hole preparation technique and diameter (P = 0.521) and pilot hole preparation technique and density (P = 0.425).
      Pearson correlation analysis showed significant correlation between torque and load only in high density block with 4.0 mm screw using drill bit (r = −0.712, P = 0.009) and in low density block with a 3.5 mm screw using a drill bit (r = 0.613, P = 0.034).

      4. Discussion

      Stable internal fixation in osteoporotic bone can be difficult to achieve.
      • Stromsoe K.
      • Kok W.L.
      • Hoiseth A.
      • et al.
      Holding power of the 4.5 mm AO/ASIF cortex screw in cortical bone in relation to bone mineral.
      As bone mineral density decreases so does screw pullout strength. Fixation becomes weaker in cancellous bone with less cortical bone thickness such as in the distal fibula or distal radius.
      • Seebeck J.
      • Goldhahn J.
      • Morlock M.M.
      • et al.
      Mechanical behavior of screws in normal and osteoporotic bone.
      It is in these extreme situations where even small increases in fixation strength may improve a patient's outcome.
      Initial screw holding strength has historically been studied with either axial pullout strength or maximal insertional torque. Pullout strength and insertional torque do not seem to be related and therefore should be measured separately.
      • Ricci W.M.
      • Tornetta 3rd, P.
      • Petteys T.
      • et al.
      A comparison of screw insertion torque and pullout strength.
      ,
      • Tankard S.E.
      • Mears S.C.
      • Marsland D.
      • et al.
      Does maximum torque mean optimal pullout strength of screws?.
      Non locking fixation techniques depend on the bone plate interface and as a result are dependent on maximal insertion torque to create an appropriate normal force for stable fixation. Locking fixation techniques are dependent on pure screw pullout strength for fixation stability as there is minimal friction at the plate and bone interface.
      The effects of the screw characteristics such as thread pitch, depth, and size on fixation stability have been well studied.
      • Ricci W.M.
      • Tornetta 3rd, P.
      • Petteys T.
      • et al.
      A comparison of screw insertion torque and pullout strength.
      ,
      • Wu Z.
      • Nassar S.A.
      • Yang X.
      Pullout performance of self-tapping medical screws.
      ,
      • Chapman J.R.
      • Harrington R.M.
      • Lee K.M.
      • et al.
      Factors affecting the pullout strength of cancellous bone screws.
      ,
      • Patel P.S.
      • Shepherd D.E.
      • Hukins D.W.
      The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models.
      The proper pilot hole and drill bit size has been evaluated by multiple authors.
      • Battula S.
      • Schoenfeld A.J.
      • Sahai V.
      • et al.
      The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone.
      ,
      • Steeves M.
      • Stone C.
      • Mogaard J.
      • et al.
      How pilot-hole size affects bone-screw pullout strength in human cadaveric cancellous bone.
      ,
      • Fincham B.M.
      • Jaeblon T.
      The effect of drill bit, pin, and wire tip design on drilling.
      ,
      • Oktenoglu B.T.
      • Ferrara L.A.
      • Andalkar N.
      • et al.
      Effects of hole preparation on screw pullout resistance and insertional torque: a biomechanical study.
      Surgeon technique, ie, screw reinsertion or overtightening has also been shown to decrease the screw fixation strength.
      • Thakkar S.C.
      • Langdale E.R.
      • Mears S.C.
      • et al.
      Can the "turn-of-the-nut" method improve cortical screw fixation?.
      ,
      • Tankard S.E.
      • Mears S.C.
      • Marsland D.
      • et al.
      Does maximum torque mean optimal pullout strength of screws?.
      ,
      • Collinge C.
      • Hartigan B.
      • Lautenschlager E.P.
      Effects of surgical errors on small fragment screw fixation.
      ,
      • Matityahu A.
      • Hurschler C.
      • Badenhop M.
      • et al.
      Reduction of pullout strength caused by reinsertion of 3.5-mm cortical screws.
      To our knowledge this is the first report to directly compare twist drill bit pilot holes with K-wire pilot holes. Our study has demonstrated that screw hole preparation with a K-wire significantly increases screw pullout strength in synthetic severely osteoporotic-type bone.
      In mildly osteoporotic-type artificial bone and with smaller screws there was no statistical difference between the K-wire and twist drill. Similarly, there was no difference in the maximal insertional torque with K-wires versus twist drills.
      Strengths of this paper include the use of identical sized twist drills and K-wires, although 2.5 mm K-wires are not found in standard orthopedic instrument trays. We controlled for screw characteristics by choosing to test 3.5 mm cortical and 4.0 mm cancellous screws only. Synthetic bone surrogate was preferred for its’ consistent bone density therefore creating reproducible results. All pullout studies were done under static load to failure which is consistent with American Society for Testing and Materials (ASTM) testing standards.
      Limitations of the paper sit mostly with the clinical applicability. Synthetic bone surrogate, although an accepted alternative to human bone, cannot replace living human bone. In addition the bone density used for the severe osteoporotic bone surrogate is at the extreme levels of human osteoporosis.
      • Abshire B.B.
      • McLain R.F.
      • Valdevit A.
      • et al.
      Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out.
      Finally, K-wire drilling is known to cause increased temperatures in cortical bone,
      • Oktenoglu B.T.
      • Ferrara L.A.
      • Andalkar N.
      • et al.
      Effects of hole preparation on screw pullout resistance and insertional torque: a biomechanical study.
      but because the amount of heat generated is proportional to the density of bone drilled,
      • Chen Y.C.
      • et al.
      Assessment of thermal necrosis risk regions for different bone qualities as a function of drilling parameters.
      ,
      • Mediouni M.
      • et al.
      Optimal parameters to avoid thermal necrosis during bone drilling: a finite element analysis.
      patients with severe osteoporosis and lower bone density are unlikely to experience heat necrosis.
      Future studies may include investigating screw pullout strength using the standard K-wires that are in most orthopaedic instrument trays, such as 1.5 mm and 2.0 mm. In addition, it would be beneficial to reproduce the study in cadaveric specimens.

      5. Conclusion

      In artificial severely osteoporotic-type bone, K-wire pilot hole preparation may improve screw pullout strength. Surgeons may consider using a K-wire rather than a drill in these situations. In mildly osteoporotic bone, K-wire pilot hole preparation does not appear to provide any benefit to standard twist drill bit preparation.

      Conflicts of interest

      Two authors receive speaking fees from Smith and Nephew. One author receives speaking fees from Trice Medical, Pacira Pharmaceuticals, Depuy Synthes, Flexion Therapeutics, and Zimmer Biomet.

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