• View in gallery

    Photograph of the 3-point bending test apparatus used to test titanium cannulated screws to failure in a study conducted to determine whether filling the guide channel of the screws with orthopedic-grade low-viscosity PMMA cement with and without the addition of a 3.2-mm-diameter stainless steel orthopedic pin enhanced the mechanical properties of the screws relative to unfilled control screws. Two 11-mm-wide rounded-tip support arms were mounted 50 mm apart to support the test specimen, and the bending arm (a) was mounted on a 5-kN load cell of a screw-driven universal testing machine such that it would impact the midpoint of the test specimen. An extensometer with a 25-mm-gauge length (b) was fixed to a spring-loaded deflectometer at the midpoint of the span between the 2 support arms. Each screw underwent monotonic testing with a loading rate of 2.5 mm/min until the specimen had an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm was achieved.

  • View in gallery

    Representative load versus displacement curves for titanium cannulated screws that were filled with PMMA cement alone (OCS group; dotted line) or in combination with a 3.2-mm-diameter orthopedic pin (PCS group; solid line) or that were left unfilled (control group; dashed line) for mechanical testing. Each screw underwent a 3-point bending test to failure as described in Figure 1. Each curve represents 1 screw from the designated group. The represented PCS screw failed owing to bending, whereas the shafts of the represented OCS and control screws broke. Consequently, there was an acute decrease in the resistance to load (indicative of the screw breaking) for the OCS and control screws but not for the PCS screw.

  • 1. Kang JS, Moon KH, Shin JS, et al. Clinical results of internal fixation of subcapital femoral neck fractures. Clin Orthop Surg 2016;8:146152.

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  • 2. Bentley VA, Edwards RB III, Santschi EM, et al. Repair of femoral capital physeal fractures with 7.0-mm cannulated screws in cattle: 20 cases (1988–2002). J Am Vet Med Assoc 2005;227:964969.

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  • 5. Nuss K. Internal fixation. In: Ducharme NG, Fubini Sl eds. Farm animal surgery. 2nd ed. St Louis: Elsevier, 2017;384400.

  • 6. Hamilton GF, Turner AS, Ferguson JG, et al. Slipped capital femoral epiphysis in calves. J Am Vet Med Assoc 1978;172:13181322.

  • 7. Marchionatti E, Fecteau G, Desrochers A. Traumatic conditions of the coxofemoral joint: luxation, femoral head-neck fracture, acetabular fracture. Vet Clin North Am Food Anim Pract 2014;30:247264.

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  • 8. Slone HS, Stacey SC, Mooney JF III. Mechanical failure of single cannulated screw fixation of unstable slipped capital femoral epiphysis. J Pediatr Orthop B 2014;23:4954.

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  • 12. Johnson NL, Galuppo LD, Stover SM, et al. An in vitro biomechanical comparison of the insertion variables and pullout mechanical properties of AO 6.5-mm standard cancellous and 7.3-mm self-tapping, cannulated bone screws in foal femoral bone. Vet Surg 2004;33:681690.

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  • 13. Hearn TC, Schatzker J, Wolfson N. Extraction strength of cannulated cancellous bone screws. J Orthop Trauma 1993;7:138141.

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Assessment of two augmentation techniques on the mechanical properties of titanium cannulated bone screws

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  • 1 1Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.
  • | 2 2Center of Renewable Carbon, University of Tennessee, Knoxville, TN 37996.

Abstract

OBJECTIVE

To determine the effects of 2 augmentation techniques on the mechanical properties of titanium cannulated bone screws.

SAMPLE

33 titanium cannulated bone screws (outer diameter, 6.5 mm; guide channel diameter, 3.6 mm).

PROCEDURES

11 screws were allocated to each of 3 groups. The guide channel of each screw was filled with polymethyl methacrylate bone cement alone (OCS group) or in combination with a 3.2-mm-diameter orthopedic pin (PCS group) or remained unmodified (control group) before mechanical testing. Each screw underwent a single-cycle 3-point bending test to failure with a monotonic loading rate of 2.5 mm/min. Failure was defined as an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm. Mechanical properties were determined for each screw and compared among the 3 groups.

RESULTS

All screws in the control and OCS groups and 1 screw in the PCS group broke during testing; a 15-mm bending deformation was achieved for the remaining 10 screws in the PCS group. Maximum load and load at failure differed significantly among the 3 groups. Stiffness and load at yield for the PCS group were significantly greater than those for the control and OCS groups but did not differ between the control and OCS groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of bone cement and an orthopedic pin to fill the guide channel of cannulated screws significantly increased the strength of the construct, but ex vivo and in vivo studies are necessary before this augmentation technique can be recommended for clinical patients.

Abstract

OBJECTIVE

To determine the effects of 2 augmentation techniques on the mechanical properties of titanium cannulated bone screws.

SAMPLE

33 titanium cannulated bone screws (outer diameter, 6.5 mm; guide channel diameter, 3.6 mm).

PROCEDURES

11 screws were allocated to each of 3 groups. The guide channel of each screw was filled with polymethyl methacrylate bone cement alone (OCS group) or in combination with a 3.2-mm-diameter orthopedic pin (PCS group) or remained unmodified (control group) before mechanical testing. Each screw underwent a single-cycle 3-point bending test to failure with a monotonic loading rate of 2.5 mm/min. Failure was defined as an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm. Mechanical properties were determined for each screw and compared among the 3 groups.

RESULTS

All screws in the control and OCS groups and 1 screw in the PCS group broke during testing; a 15-mm bending deformation was achieved for the remaining 10 screws in the PCS group. Maximum load and load at failure differed significantly among the 3 groups. Stiffness and load at yield for the PCS group were significantly greater than those for the control and OCS groups but did not differ between the control and OCS groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of bone cement and an orthopedic pin to fill the guide channel of cannulated screws significantly increased the strength of the construct, but ex vivo and in vivo studies are necessary before this augmentation technique can be recommended for clinical patients.

Orthopedic screws are routinely used in orthopedic surgeries. Cannulated screws provide surgeons the ability to selectively position screws before insertion into bone to improve the accuracy of placement. In human surgery, large-diameter cannulated screws are commonly used for internal fixation of femoral neck fractures.1 Large-diameter cannulated screws are also used for repair of femoral capital fractures in cattle.2–5 Femoral capital fractures occur most commonly in newborn calves as a result of dystocia and forced-extraction delivery and in older cattle as a result of trauma caused by falls or herd mates, especially in group-housed bulls.6,7 Treatment of such fractures requires internal fixation of the femur by either diverging pins or the insertion of screws in lag fashion to compress the fracture site.3,4,6 The divergent orientation of the implants counteracts rotational forces during the weight-bearing phases of a stride. The femoral head and neck are located on the medial aspect of the bone at an angle to the femoral shaft. Therefore, axial loading of the pelvic limb during the weight-bearing phase of a stride concentrates forces on the femoral head and neck. During weight bearing, tension is applied to the dorsal aspect of the femoral neck and compression is applied to the ventral aspect of the femoral neck. Implants used to stabilize femoral head and neck fractures need to counteract those forces so the animal can ambulate during the postoperative period. Bending is the most common mode of failure for implants used to stabilize femoral head and neck fractures in both human patients and cattle.2,8,9

Cannulated screws allow surgeons to insert a guide pin into the bone and verify its position before a larger hole is drilled into the bone to accommodate the screw. Small bone fragments are not conducive to multiple attempts to attain adequate screw placement; therefore, precise screw placement is crucial during repair of such fractures. Cannulated screws are manufactured with various materials including stainless steel, titanium, and degradable polymeric material.10 Cannulated screws made from stainless steel have the greatest resistance to bending but are less resistant to deformation from cyclical loading. Titanium cannulated screws have the greatest resistance to deformation from cyclical loading but are less resistant to bending. Polymeric cannulated screws have the advantage of being absorbable but provide little mechanical strength. Surgeons must select orthopedic implants on the basis of the unique needs of each patient. Ideally, surgical implants are chosen on the basis of their ability to resist the forces applied to them. Titanium cannulated screws are being increasingly used in orthopedic surgery in both human and veterinary medicine. However, the cannulated nature of those screws introduces weakness to the repair because the yield point, breaking point, and bending moment of cannulated screws are inferior to those of solid screws.11 Insertion variables and pullout mechanical properties of 7.3-mm-diameter cannulated screws have been described for femoral bones of foals12–14 as have the shear characteristics of 6.5-mm-diameter stainless steel cannulated screws.15 Threaded inserts have been used to reinforce 4-mm-diameter cannulated screws.16

In our hospital, we maintain titanium cannulated screws as an option for fracture repair in select patients. However, we are cognizant of the inherent weakness of those screws and are constantly investigating methods to enhance the resistance of titanium cannulated screws to bending forces and thereby decrease the risk of postoperative implant failure. The purpose of the study reported here was to determine the mechanical properties of titanium cannulated screws by use of a 3-point bending test and to evaluate whether those mechanical properties could be enhanced by filling the cannulated screws with material readily available to orthopedic surgeons such as PMMA bone cement alone or in combination with an orthopedic pin. We hypothesized that filling the guide channels of cannulated screws would increase the stiffness and strength of those screws relative to standard (unfilled) cannulated screws.

Materials and Methods

Screws

Thirty-three titanium cannulated screwsa (shank type; 22-mm end threaded; outer diameter, 6.5 mm; guide channel diameter, 3.6 mm; and length, 80 to 120 mm) were allocated to 3 test groups (11 screws/group) prior to mechanical testing. The guide channels of the screws in the OCS group were filled with orthopedic-grade low-viscosity PMMA cement.b The guide channels of the screws in the PCS group were filled with the same PMMA cement as the screws in the OCS group in addition to a 3.2-mm-diameter stainless steel orthopedic pin,c which was inserted into the cement before it hardened. The guide channels of the screws in the control group remained unfilled.

For the OCS and PCS groups, PMMA cement was mixed in accordance with label instructions. After a uniform consistency for the cement was achieved, the cement was drawn into syringes and immediately injected into the guide channel of each designated screw beginning at the screwhead and continuing until cement exited the tip of the screw. Then, for each screw in the PCS group, a 3.2-mm-diameter stainless steel orthopedic pin was inserted into the guide channel through the unset PMMA cement. The PMMA cement was allowed to cure for at least 2 hours before mechanical testing was initiated.17

Mechanical testing

Each screw underwent a single-cycle 3-point bending test to failure. The 3-point bending test apparatus was designed with two 5-mm-wide support arms mounted 50 mm apart to support the test specimen and a 5-mm-wide bending arm positioned to impact the midpoint of the test specimen. The bending arm was mounted on a 5-kN load cell of a screw-driven universal testing machine.d Bending deflection was measured by a spring-loaded deflectometer positioned at the center point of the span between the support arms. An extensometer with a 25-mm-gauge length was fixed to the deflectometer (Figure 1). Monotonic testing was performed with a loading rate of 2.5 mm/min as described17 until the specimen had an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm (ie, maximal displacement) was achieved.

Figure 1—
Figure 1—

Photograph of the 3-point bending test apparatus used to test titanium cannulated screws to failure in a study conducted to determine whether filling the guide channel of the screws with orthopedic-grade low-viscosity PMMA cement with and without the addition of a 3.2-mm-diameter stainless steel orthopedic pin enhanced the mechanical properties of the screws relative to unfilled control screws. Two 11-mm-wide rounded-tip support arms were mounted 50 mm apart to support the test specimen, and the bending arm (a) was mounted on a 5-kN load cell of a screw-driven universal testing machine such that it would impact the midpoint of the test specimen. An extensometer with a 25-mm-gauge length (b) was fixed to a spring-loaded deflectometer at the midpoint of the span between the 2 support arms. Each screw underwent monotonic testing with a loading rate of 2.5 mm/min until the specimen had an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm was achieved.

Citation: American Journal of Veterinary Research 81, 2; 10.2460/ajvr.81.2.116

A load-versus-displacement curve was generated for each screw. Load and displacement at yield, maximum load, and failure were measured, and stiffness was calculated by testing machine software.e Yield was defined as the 0.2% strain offset of the linear portion load. Mode of failure (breakage or bending) was also recorded for each screw.

Statistical analysis

Outcomes of interest were stiffness and load and displacement at yield, maximum load, and failure as well as mode of failure. A Fisher exact test was used to compare the mode of failure among the 3 test groups (OCS, PCS, and control). The effect of test group on each continuous variable of interest was assessed by means of a 1-way ANOVA, with the Tukey method used for post hoc pairwise comparisons when necessary. To verify that ANOVA model assumptions were met, the Shapiro-Wilk test was used to assess the distribution of residuals for normality and the Levene test was used to assess residuals for homoscedasticity. All analyses were performed with a commercially available statistical software program,f and values of P < 0.05 were considered significant.

Results

Mode of failure

For all screws in the OCS and control groups, the shaft broke acutely before a displacement of 15 mm was achieved (Figure 2). Ten of the 11 screws in the PCS group reached maximal displacement (15 mm) without breaking (ie, the mode of failure was bending), whereas the shaft of the remaining screw broke at a displacement of 10.58 mm. The mode of failure for the screws in the PCS group differed significantly (P = 0.001) from that for the screws in the OCS and control groups. The mode of failure did not differ significantly between the screws in the OCS and control groups.

Figure 2—
Figure 2—

Representative load versus displacement curves for titanium cannulated screws that were filled with PMMA cement alone (OCS group; dotted line) or in combination with a 3.2-mm-diameter orthopedic pin (PCS group; solid line) or that were left unfilled (control group; dashed line) for mechanical testing. Each screw underwent a 3-point bending test to failure as described in Figure 1. Each curve represents 1 screw from the designated group. The represented PCS screw failed owing to bending, whereas the shafts of the represented OCS and control screws broke. Consequently, there was an acute decrease in the resistance to load (indicative of the screw breaking) for the OCS and control screws but not for the PCS screw.

Citation: American Journal of Veterinary Research 81, 2; 10.2460/ajvr.81.2.116

Biomechanical properties

Stiffness and load and displacement at yield, maximum load, and failure were summarized for the 3 test groups (Table 1). The mean ± SD stiffness for the PCS group (108,368 ± 6,638 N/mm2) was significantly greater than that for the OCS (77,826 ± 3,865 N/mm2) and control (76,297 ± 2,486 N/mm2) groups. The mean stiffness did not differ significantly between the OCS and control groups.

Table 1—

Mean ± SD values for select mechanical properties of titanium cannulated screws (outer diameter, 6.5 mm; guide channel diameter, 3.6 mm) that were filled with PMMA cement alone (OCS group; n = 11) or in combination with a 3.2-mm-diameter orthopedic pin (PCS group; 11) or that were left unfilled (control group; 11) for mechanical testing.

PropertyEventOCS groupPCS groupControl group
Stiffness (N/mm2)77,826 ± 3,865a108,368 ± 6,638b76,297 ± 2,486a
Load (N)Yield1,021.84 ± 142.92a1,204.38 ± 163.83b978.81 ± 52.34a
 Maximum load1,438.50 ± 44.50a2,064.57 ± 60.96b1,325.26 ± 47.21c
 Failure1,338.60 ± 40.13a1,898.45 ± 64.82b1,248.34 ± 68.93c
Displacement (mm)Yield1.62 ± 0.88a1.28 ± 0.19a1.37 ± 0.10a
 Maximum load6.99 ± 0.44a8.38 ± 0.29b5.83 ± 0.48c
 Failure9.79 ± 1.84a14.93 ± 1.49b8.35 ± 1.02a

Each screw underwent a 3-point bending test to failure; a monotonic loading rate of 2.5 mm/min was applied until the specimen had an acute decrease in resistance to load of ≥ 20% or a bending deformation of 15 mm was achieved. – = Not applicable.

Within a row, values with different superscript letters differ significantly (P < 0.05).

The mean displacement at yield did not differ significantly among the 3 test groups. However, similar to stiffness, the mean ± SD load at yield for the PCS group (1,204.38 ± 163.83 N) was significantly greater than that for the OCS (1,021.84 ± 142.92 N) and control (978.81 ± 52.34 N) groups but did not differ significantly between the OCS and control groups.

The load at maximum load differed significantly among the 3 test groups and was greatest for the PCS group (2,064.57 ± 60.96 N) and lowest for the control group (1,325.26 ± 47.21 N). The mean ± SD displacement at maximum load differed significantly among the 3 test groups and was greatest for the PCS group (8.38 ± 0.29 mm), compared with that for the OCS (6.99 ± 0.44 mm) and control (5.83 ± 0.48 mm) groups.

The mean load at failure was less than the mean load at maximum load for all 3 test groups. The mean load at failure differed significantly among the 3 groups and was greatest for the PCS group (1,898.45 ± 64.82 N) and lowest for the control group (1,248.34 ± 68.93 N). The mean displacement at failure for the PCS group (14.93 ± 1.49 mm) was significantly greater than that for the OCS (9.79 ± 1.84 mm) and control (8.35 ± 1.02 mm) groups but did not differ significantly between the OCS and control groups.

Discussion

Anatomic reconstruction of fractures is important for the creation of a load-sharing construct among the components, including bone and orthopedic implants. Bone screws provide a means of connecting bone segments, either through positioning and alignment or by the use of lag-screw techniques to compress bone fragments together. The stability of a screw-fracture repair is dependent on reduction and alignment of bone fragments and interfragmentary compression. Bone screws can fail because of excessive acute loading (single-cycle failure) or fatigue (cyclical loading). Screws must be able to withstand acute loading before their ability to withstand cyclical loading can be evaluated. Screw failure is generally the result of breakage, bending, or pullout. Bending of cannulated screw implants is a clinically relevant complication because it alters the reduction and stability of the fracture repair and may necessitate revision of the repair.8

Cannulated screws are particularly well suited as implants for accurate and precise reduction and reconstruction of fractures, especially when bone fragments are not readily assessible, such as fractures of the head and neck of the femur. However, the guide channel through the center of a cannulated screw lessens its resistance to bending. Techniques to improve the resistance of cannulated screws to bending after implantation might be beneficial in preventing postoperative implant failure. Orthopedic construct augmentation by injection of bone cement into the holes of laterally perforated cannulated screws has been described in osteoporotic bone,18–20 and although such augmentation enhances anchoring of the implant to the bone, it does not enhance the mechanical properties of the implant itself.

The addition of metal inserts on the mechanical properties of cannulated screws has been evaluated in multiple studies.16,21 In a study by Shih et al,21 the insertion a 2-mm-diameter orthopedic pin into a 6.5-mm-diameter cannulated spinal pedicle screw (guide channel diameter, 2 mm) did not significantly affect the yield load and stiffness of the construct relative to that for unmodified (control) cannulated spinal pedicle screws. In that study,21 the orthopedic pin was assumed to be in contact with the guide channel throughout the length of the screw, given that both had a diameter of 2 mm. In a study by Dundon et al,16 the addition of a metal threaded insert into the guide channel of a customized 4.0-mm-diameter cannulated screw (guide channel diameter, 2 mm) significantly increased the yield and maximum load of the construct, compared with that for control cannulated screws without the insert. The threaded insert was locked, or firmly connected, to the cannulated screws of the Dundon et al16 study, whereas the orthopedic pins were simply in contact with the guide channel of the cannulated spinal pedicle screws of the Shih et al21 study, and that difference may have contributed to the apparently conflicting results between the 2 studies. For the PCS construct evaluated in the present study, the orthopedic pin was adhered to the guide channel of a cannulated screw by bone cement, which prevented the pin from sliding within the guide channel during mechanical testing.

In the Dundon et al16 study, the mean ± SD load at failure for unmodified 4.5-mm-diameter stainless steel cannulated screws with a 2-mm-diameter guide channel (1,804.68 ± 29.73 N) was significantly greater than that for unmodified 6.5-mm-diameter titanium cannulated screws with a 3.6-mm-diameter guide channel (1,325.27 ± 47.17 N). That difference reflected the variations between the mechanical characteristics of stainless steel and titanium. However, the span length of the testing system was not specified in that study,16 and differences in span length could have contributed to the differences in load at failure observed between the 2 types of screws. In the present study, only titanium cannulated screws were evaluated, and a span length of 50 mm was used for the 3-point bending tests, which resulted in a span length-to-screw diameter ratio of 10. In clinical patients and dependent on the extent of anatomic reduction of the fracture achieved, stress on the implant is generally concentrated over a distance < 50 mm, which would lead to increased localized stress and perhaps shear forces on the implants, compared with the experimental conditions to which the screws of the present study were subjected.

For the PCS construct evaluated in the present study, the addition of a stainless steel pin to the guide channel of titanium cannulated screws in combination with PMMA cement significantly enhanced the strength and stiffness of the construct relative to unmodified (control) screws and screws in which the guide channel was filled with PMMA cement only. Those findings were contradictory to the results of the aforementioned Shih et al21 study. The simple addition of a solid core insert into the guide channel of a cannulated screw may not provide sufficient enhancement along the length of the screw to increase the load at yield and stiffness of the implant. Moreover, the addition of a pin without bone cement to fill the voids between the pin and screw might have been limited by focal resistance at the location of bending with the remaining length of the insert (pin) sliding within the guide channel.21 It is also possible that the screw passed the yield point before the non-secured pin was able to withstand the load. The addition of PMMA cement to the construct eliminated any voids between the screw and pin, and thereby improved the distribution of force along the entire length of the implant so that both the PMMA cement and pin contributed to the mechanical properties of the reinforced screw.

The mode of failure of the PMMA cement within the guide channels of the cannulated screws could not be determined in the present study. We suspected that the brittleness of the PMMA cement might have resulted in cracking early during the bending process, which limited differences in the modulus and yield for the PCS group. The modulus of elasticity is 24,600 kp/cm2 (2,412 N/mm2) for a 40-mm2 rectangular cross section of surgical PMMA cement tested in bending.22 The high modulus of elasticity (ie, inelasticity) for PMMA cement might have contributed to the limited augmentation of mechanical properties observed for the OCS group relative to the control group. For the PCS group, the pins may have been secured in place by the PMMA cement initially and then by focal resistance at the site of bending, which enhanced the strength of that construct relative to the OCS construct.

The use of bone cement for reinforcement of orthopedic constructs in osteoporotic bone has been described.23 In that study,23 the experimental model consisted of human osteoporotic femurs (n = 7 pairs [both femurs from the same subject]), a large titanium locking compression bone plate, and 7 cannulated and perforated locking bone screws. For each pair of femurs, the screws of the orthopedic implant were augmented with bone cement for one and left unmodified (control) for the other.23 For the augmented constructs, bone cement was injected through cannulated and perforated screws such that it filled the guide channel and perforations and extruded into the surrounding bone to improve the anchorage between the implant and bone and decrease the effect of osteoporosis on the stability of the bone-implant interface.23 All constructs underwent single cycle-to-failure testing.23 Results indicated that the bone cement was the weakest part of the construct, and the stiffness did not differ significantly between the augmented and control groups.23 That experimental model was substantially different from that used in the present study. In the present study, bone cement was evaluated as augmentation for stand-alone implants (screws) in an in vitro experiment. Results indicated that, under the loading conditions evaluated, bone cement in combination with an orthopedic pin, but not bone cement alone, significantly enhanced the strength of titanium cannulated screws.

For the screws in the OCS and PCS groups of the present study, PMMA cement was injected into the guide channel of each screw until it overflowed onto the outer tip of the screw. For clinical patients, the volume of PMMA cement to be infused into a cannulated screw can be estimated by calculating the volume of the guide channel (channel diameter × screw length × π). Bone porosity will also affect the volume of cement that can be injected into a cannulated screw. The injection of bone cement into the guide channel of a cannulated screw might be challenging in situ because air may be impeded from escaping from the channel as it is displaced by the cement, thereby increasing the pressure within the channel. When stainless steel orthopedic pins are used in combination with PMMA cement to augment cannulated screws, the pin should be the same length as the screw and should be fully inserted into the guide channel immediately after the cement is injected. Displaced cement will exit the hole in the screwhead and should be promptly removed to prevent its extrusion into surrounding tissues. Further research is necessary before this technique is recommended for clinical patients.

Mechanical properties of implants may differ depending on whether the implants undergo testing in a single cycle-to-failure or cyclical-loading manner. Results of the Dundon et al16 study indicate that cannulated implants had a lower load to failure and less resistance to fatigue than did solid-core screws. In the present study, the mechanical properties for each of 3 cannulated screw constructs were determined only by single cycle-to-failure testing; cyclic testing would be necessary to evaluate the effect of implant construct on resistance to fatigue.

The use of implants made from different metals in a single orthopedic repair is discouraged because the electrochemical potential difference between different metal alloys can trigger galvanic corrosion. The PCS construct evaluated in the present study included a titanium cannulated screw and stainless steel orthopedic pin. We hypothesized that the PMMA cement would be sufficiently distributed around the pin in the guide channel to maintain some degree of separation between the 2 metals and thereby decrease the risk of corrosion. However, limited contact between the pin and the screw was possible and should be considered before that construct is used in clinical patients. Results of an in vitro study24 indicate that the amount of metal released into the surrounding environment did not differ between constructs consisting of bone plates and screws of different metals and constructs consisting of bone plates and screws of the same metal. Nevertheless, use of a single metal in orthopedic implant constructs is recommended for clinical applications.

The present study was conducted because we were interested in exploring methods to enhance the mechanical performance of cannulated bone screws. We chose to evaluate screws made from titanium, a metal that is fairly resistant to fatigue but not so resistant to acute loading, because we maintain an inventory of such screws at our hospital for use in clinical patients. Given the known properties of titanium, we elected to assess the effect of screw augmentation on properties associated with acute loading rather than those associated with resistance to fatigue. Although the magnitude of effect that each augmentation technique had on the mechanical properties of titanium cannulated screws cannot be translated to cannulated screws made from stainless steel, the underlying mechanism for that effect is expected to be applicable across materials. Further research is necessary to evaluate whether the nature of the augmentation-induced alterations in the mechanical properties of cannulated screws observed in the present study remain consistent regardless of screw material and size and loading conditions. In the meantime, orthopedic surgeons must make decisions regarding implant constructs on a case-by-case basis within the context of the unique needs of each patient and their skills and facilities.

Results of the present in vitro study indicated that filling the guide channel of titanium cannulated bone screws with PMMA cement in combination with an orthopedic pin increased the strength and stiffness of the resulting constructs. Orthopedic pins and PMMA cement are commonly available to orthopedic surgeons. Additional research is necessary to determine whether the sequential injection of PMMA cement followed by insertion of an orthopedic pin into the guide channel of a cannulated screw is achievable in ex vivo and in vivo settings before the implant augmentation techniques described in this study can be recommended for use in clinical patients.

Acknowledgments

The authors thank Dr. Sun for assistance with statistical analysis.

ABBREVIATIONS

PMMA

Polymethyl methacrylate

Footnotes

a.

Ace Medical Co, Los Angeles, Calif.

b.

Surgical Simplex P, Stryker, Mahwah, NJ.

c.

Imex, Longview, Tex.

d.

Instron, Norwood, Mass.

e.

Bluehill 3, Instron, Norwood, Mass.

f.

SAS, version 9.4, SAS Institute Inc, Cary, NC.

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Contributor Notes

Dr. Zarzosa's present address is Austin Veterinary Animal Clinic, 1550 Lennoxville Rd, Beaufort, NC 28516.

Address correspondence to Dr. Mulon (pmulon@utk.edu).