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The effect of extracorporeal shock wave lithotripsy on the prosthesis interface in cementless arthroplasty
The Effect of Extracorporeal Shock Wave Lithotripsy on t h e Prosthesis Interface in C e m e n t l e s s Arthroplasty Evaluation in a Rabbit M o d e l
Steven K. S t r a n n e ,
MD,
Christian John
John
J. C a l l a g h a n ,
S. F u l g h u m ,
L. W e i n e r t h ,
MD, MD,
MD, Thomas
Richard
R. G l i s s o n ,
and Anthony
M. Fyda, MD, BS,
V. S e a b e r
Abstract: The effect of extracorporeal shock wave lithotripsy on interfacial strength between prosthesis and bone in cementless arthroplasty was examined using a rabbit model. Paired femora, each implanted with fiber mesh porous coated titanium implants, were harvested from rabbits 15 weeks after implantation. In group I, one femur from each pair was exposed to lithotripsy treatment consisting of 2,000 shocks at 20 kV. In group II, one femur from each pair was exposed to 2,000 shocks at 26 kV. Contralateral femora from each pair served as controls in both groups. Mechanical pushout tests were conducted on the implants using a 1321 tnstron testing machine at a constant rate of 1 ram/minute. Shock waves generated at 20 kV were found to have no significant decrease on either the prosthesis/bone interfacial strength or energy to failure of cementless implants. Shock waves generated at 26 kV produced a mean 17.45% decrease in the prosthesis/bone interfacial strength, which approached statistical significance (P = .062), and a 7.84% mean decrease in the energy to failure (P = .268). However, in four of the seven group II specimens, cortical fractures occurred. These findings suggest that lithotripsy will not aid in the removal of uncemented porous coated devices and lithotripsy inadvertently focused at an uncemented device will not disrupt significantly the prosthesis-bone interface. K e y words: lithotripsy, uncemented, prosthesis, rabbit.
A n u m b e r of investigators h a v e evaluated the use of extracorporeal shock w a v e lithotripsy ~~.~3,~8-2~ to facilitate the r e m o v a l of c e m e n t that is m e c h a n i c a l l y i n t e r l o c k e d w i t h b o n e . The effectiveness of litho
tripsy to disrupt the c e m e n t - b o n e interface has b e e n attributed to the acoustic i m p e d a n c e difference bet w e e n b o n e a n d b o n e cement. 2°,2t At a n interface b e t w e e n t w o materials, the force m a g n i t u d e delivered by a shock w a v e increases p r o p o r t i o n a t e l y w i t h the difference in a c o u s t i c i m p e d a n c e b e t w e e n t h e t w o materials.2,10,x6 A l t h o u g h n o investigations of the potential for high e n e r g y s h o c k w a v e s to disrupt the p r o s t h e s i s - b o n e interface in c e m e n t l e s s a r t h r o p l a s t y h a v e b e e n p e r f o r m e d , the a c o u s t i c
From the Division of Orthopaedic Surgery, Duke University Medical Center, Durham, North Carolina. Supported by the Zimmer Corporation and the Piedmont Othopaedic Society. Reprint requests: John J. Callaghan, MD, Department of Orthopaedics, University of Iowa, Iowa City, IA 52242.
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impedance difference between bone and metal is significantly greater than the difference between bone and p o l y m e t h y l m e t h a c r y l a t e . Hence, lithotripsy could theoretically aid in disruption of the prost h e s i s - b o n e interface for r e p l a c e m e n t of uncem e n t e d prostheses even m o r e effectively t h a n the disruption of the c e m e n t - b o n e interface in cemented replacements. In addition to the potential efficacy of lithotripsy to facilitate u n c e m e n t e d revision arthroplasty, the safety of lithotripsy treatment for kidney and gallbladder calculi in patients with u n c e m e n t e d prostheses has been questioned but not addressed. Areas well outside of the lithotripter's focus are subjected to significant shock wave energy, as evidenced by the report of an inadvertent gallstone fracture secondary to shock wave treatment aimed at a kidney stone, x5 Acetabular prostheses and spinal instrumentation m a y come within centimeters of kidneys, ureters, gallbladders, and other lithotripsy targets. Thus, the potential effect of clinical lithotripsy treatment on the fixation of u n c e m e n t e d prostheses requires definition. This investigation is designed to examine the effect of extracorporeal shock wave lithotripsy on the prost h e s i s - b o n e interface in a cementless porous ingrowth arthroplasty model.
Materials and Methods Forty-two implants were placed bilaterally in the distal femoral metaphyses of 21 New Zealand white rabbits weighing 4 . 0 - 6 . 0 kg. The cylindrical implants (Zimmer, Warsaw, IN) were covered circumferentially with commercially pure titanium mesh porous coating. Cylinder dimensions were 4.6 m m in diameter and 9.1 m m in length. Implantation and subsequent mechanical testing in this rabbit model were performed as described by Lavernia et al. ~2 A 2 cm lateral incision exposed the distal femoral metaphysis. In each femur, a 5 m m drill hole was centered 1 m m proximal to the insertion of the lateral collateral ligament. Drill holes were oriented through the lateral cortex toward the medial cortex. The implants were pressed into the drill holes and soft tissues were closed with suture. Implantation was conducted using sterile technique. Postoperatively, the rabbits were returned immediately to unrestricted weight-bearing activity. A 7 day course of prophylactic t r i m e t h o p r i m - s u l f a m e t h o x a z o l e was administered by intramuscular injection. Fifteen weeks after the surgical implantation procedure, radiographs of the hind limbs were acquired
and the rabbits were killed. Bilateral femora were harvested and stripped of all soft tissues, including periosteum. Two groups consisting of,eight pairs each w e r e designated for mechanical study and one femur from each pair was selected for lithotripsy treatment. In both groups, the contralateral m e m b e r of each pair served as a paired control and received n o shock wave exposure. Each f e m u r to receive lithotripsy treatment was suspended by a specially designed rig in the water bath of the Dornier HM 3 lithotripter used clinically in this hospital. Shock waves were oriented sequentially from four different directions (Fig. i) and were targeted o n the bone-prosthesis interface. Group I femora received a total of 2,000 shocks administered in four groups of 500 at 20kV. Group II femora received a total of 2,000 shocks in four groups of 500 at 26 kV. The contralateral control femur of each pair did not receive exposure to lithotripsy but was soaked in a water bath for an equivalent time period. An additional five pairs of implants were tested mechanically and neither side received lithotripsy treatment in order to validate side-to-side paired comparison in this model. All femora were prepared identically for pushout testing. Bone tissue obstructing implant ends was removed with a high-speed burr. T h r o u g h o u t preparation and testing, the specimens remained at r o o m temperature and were kept moist with saline-soaked gauze to maintain the fully hydrated state. All specimens were tested within 8 hours of sacrifice. In mechanical pushout tests, each cylindrical implant was precisely aligned with the axis of test of a 1321 Instron testing machine. The diaphysis of the bone was m o u n t e d in polymethylcrylate and the distal edge of the condyle was supported with a metal
Fig. 1. Orientation of the four groups of shock waves targeted on the implants.
ESWL and Prosthesis Interface
Fig. 2. A specimen prepared for pushout testing in the Instron testing machine. brace to eliminate bending (Fig. 2). Mechanical axial pushout tests were performed at a constant rate of 1 mm/minute. Force versus deflection data were obtained and stored digitally on computer hard disk. The data were calibrated and used to calculate maxim u m force and strain energy to failure. Force of pushout was defined as the m a x i m u m force achieved during the test. Energy capacity was defined as the integral of the force versus displacement plot from zero displacement to the point of m a x i m u m force. To eliminate the effect of interspecimen variability, paired specimens and paired statistical analyses (oneway ANOVA) were used. In addition to mechanical testing, scanning electron microscopy was used to examine the surrounding bone and the retrieved specimen following pushout testing in one pair from both experimental and control groups (four specimens).
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discarded. In another pair, which was discarded, the core of the plug separated from the porous coating during pushout testing, representing poor sintering of the coating to the plug. Hence, for this report there were seven successfully treated pairs in each group. In every case, failure occurred at the i m p l a n t - b o n e interface and was not attributable to failure of the titanium fiber coating. Side-to-side comparison in untreated paired implants to validate the p u s h o u t testing m e t h o d revealed no statistical differences between sides (P = .28 by paired t-test) in force and energy capacity required to pushout. The m e a n force required for mechanical pushout tests is expressed as a percentage difference from the contralateral control of each pair (Fig. 5). Group I femora, exposed to 2,000 shocks at 20 kV, revealed no significant decrease in pushout strength or energy to failure as compared to controls. Only one pair demonstrated a decrease of 10% or greater than control for either parameter measured. Group II femora, exposed to 2,000 shocks at 26 kV, revealed a reduction in m e a n mechanical pushout strength of 17.45 %, which approached statistical significance (P = .0625) and a reduction in energy to
Results In all cases, radiographic evaluation revealed no lucencies at the implant interface and was consistent with significant bone ingrowth (Fig. 3). Gross examination of harvested femora revealed all implants to be stable without sign of fibrous encapsulation, infection, or inflammatory reaction. Following lithotripsy treatment, all implants remained grossly stable. Four of the seven lithotripsytreated femora in group II revealed grossly evident cortical fractures following treatment. This consisted of 1 0 - 2 0 m m fissures extending from the plug to the bone shaft in three specimens and a 3 x 8 m m defect created in the shaft of the fourth (Fig. 4). One of the bones in one pair from each of the groups fractured at the mounting interface during mechanical testing. In both cases the entire pair was
Fig. 3. Radiograph of implants following 15 weeks of ingrowth revealing no sign of loosening or infection.
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Fig. 4. Cortical fracture (white arrow) was grossly evident in two group II specimens.
failure of 7.84% (P -= .268). Six of the seven pairs in this group displayed a 10% or greater reduction in both force and energy. Scanning electron microscopy (Fig. 6) demonstrated no surrounding bone microfractures. However, as documented, gross fractures did occur in some group II experimental specimens. Scanning electron microscopy demonstrated similar findings in the control and experimental group pullout specimens. Failure occurred at the surrounding bone interface with some bone still attached to the porous plugs (Fig. 6B) in all specimens.
Discussion The specific aims of this investigation were to evaluate the use of extracorporeal shock wave lithotripsy applied to facilitate porous coated cementless arthroplasty revision and to evaluate any adverse effects o n the fixation of cementless implants by lithotripsy treatment aimed at renal or gallstone targets, This evaluation of the bone-prosthesis interface following shock wave exposure was considered because of the reported success of the lithotripter to disrupt the c e m e n t - b o n e interface 1~'x3"2°'21 and because of
120 100 % of 80 Control 60 40 20 0
force
energy
force
energy
20kV 26kV Fig. 5. Mean and standard deviation of the pushout force and energy to failure for group I (input voltage of 20 kV) and group II (input voltage of 26 kV) expressed as a percentage of paired contralateral control (N = seven pairs in each group).
the larger acoustic i m p e d e n c e m i s m a t c h b e t w e e n metal and bone than between cement and bone. A linear relationship exists between input kilovoltage and the pressure created at the focus characterized by the e q u a t i o n pressure psi = 0.347 kV - 0.669. 9 This study employed shock waves created by input voltages of either 20 kV or 26 kV, representing both moderate and high shock wave magnitudes. For renal stone comminution, voltages between 18 and 24 kV are c o m m o n l y employed for 1,200-1,800 shocks. In previous studies of cemented arthroplasty models, input voltages of 20 kV 2° and 25 kV ~3 have been s h o w n to decrease c e m e n t - b o n e interfacial strength significantly. To have clinical significance, the changes in interfacial ( b o n e - p r o s t h e s i s in this case) strength produced by a lithotripter to facilitate revision arthroplasty or to loosen a clinically secure prosthesis (from a misdirected focus) must be large (40% or greater). Based on side-to-side comparisons in our paired untreated controls, the sensitivity displayed in this mechanical pushout model allows detection of changes of this magnitude. Several additional factors idealize the shock wave pressures achieved at the bone-prosthesis interface and serve to increase the sensitivity of this model to detect disruption of the bone-prosthesis interface. The absence of soft tissues and the relatively small dimensions of the rabbit femora result in less attenuation of the shock wave energy than in the h u m a n clinical condition. In addition, the overall size of the implant allowed the e n t i r e implant to be placed within the focus of the HM 3 Dornier lithotripter. The implant would never fall directly in the focus during the clinical treatment of renal or gallbladder calculi. In addition, placing the entire implant w i t h i n the focus idealizes the clinical condition for facilitation of revision arthroplasty. The factors in this m o d e l maximize the shock wave pressures achieved at the i m p l a n t - b o n e interface. Multiple investigators have evaluated p u s h o u t tests to access the bone-prosthesis interface in po-
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Fig. 6. Scanning electron microscopy revealed no microfractures in the surrounding bone (A) and a thin layer of bone attached to the porous surface of the retrieved specimens (B).
rous coated implants. L3-5,7, 24, ~7 Pushout testing provides an effective m e a n s of comparing the relative interracial strength w h e n g e o m e t r y and load regim e n s are identical. 8 Potential sources of error include alignment of the implant with axis of test, taper or irregularity of the porous coated implant, and variability in the quality a n d q u a n t i t y of i n g r o w t h in paired specimens. An attempt was m a d e to minimize these variables in this investigation by modifying a previously used jig,22 which accurately aligns the implant with the axis of test and uses specifically designed and m a n u f a c t u r e d cylindrical plugs (Zimmer, Warsaw, IN). In addition, the specimens were tested soon after sacrifice, were maintained fully hydrated, and were not frozen. The m e a n values, standard deviations, and standard errors of the m e a n found in the present study correlate well with those reported previously. ~,3-5,7,14, 2 7 Scanning electron microscopy was used both to validate b o n y ingrowth in this rabbit model and to examine specimens after testing. Significant differences in cortical or trabecular microfractures were not detected either within groups or between groups.
However, group II (26 kV) specimens revealed overt fractures in four of seven specimens. Failure in the experimental and control groups in p u s h o u t testing occurred b e t w e e n the surrounding b o n e and a thin layer of bone attached to the porous surface (Fig. 6). Based on this investigation, extracorporeal shock w a v e lithotripsy is not useful for facilitating the revision of u n c e m e n t e d implants. This model provided a best-case scenario for disrupting the p r o s t h e s i s - b o n e interface because of minimal shock w a v e attenuation and placement of the entire implant within the lithotripter focus. H o w e v e r , in n e i t h e r g r o u p w a s the m e a n reduction in p u s h o u t strength great e n o u g h to warrant use of this technology. In group I (20kV), only one of the seven pairs exhibited a strength reduction of greater than 10% of control. This is in contrast to a cemented arthroplasty model, for w h i c h a reduction of 63% in interfacial strength was reported by Weinstein using an input voltage of 20 kV. 2° In group II pairs (26 kV), all but one of the treated pairs showed a reduction in both strength and energy to failure. Statistically, a trend toward decreased p u s h o u t force to failure (P = .0620) was
178 The Journal of Arthroplasty Vol. 7 No. 2 June 1992 demonstrated. However, the m e a n magnitude of reduction was small and probably of minimal conseq u e n c e for the clinical revision of prostheses. Because of the cortical fractures generated at this shock wave magnitude level, increasing the input voltage is inappropriate. Acetabular prostheses and spinal instrumentation m a y come within centimeters of the focus during lithotripsy treatment of urologic or gallbladder calculi. Of interest is whether either cementless implants will loosen w h e n subjected to lithotripsy or the presence of the metal implant will predispose the bone to fracture. Although the lithotripter was focused directly o n the implant, 2,000 shocks at 20 kV showed no effect on the strength of the u n c e m e n t e d implant interface in this investigation. The strength reduction seen in group II (26 kV) specimens was both small and exaggerated in this model due to direct focus on the implant. Pressures generated by shock w a v e s are greatest at the center of focus, and the pressure magnitude falls by more than 80% at a position 1.5 cm from the axis of focus. 9'16 In this rabbit model, bone fractures were created by 26 kV shock waves in femora containing metallic implants. While the thin cortex of the rabbit provides a sensitive m e a s u r e of such damage, fracture of h u m a n bones (without implants) exposed to shock waves targeted directly o n the b o n e has been reported. Fracture through the cortex of a flesh cadaveric h u m a n femur resulting from a single shock wave (25 kV) targeted directly at the bone 6 and the disruption of trabeculae in vertebrae subjected to several such shocks has been reported. 6,m However, no observable fractures of ribs or other bones have been demonstrated t h r o u g h o u t the significant in vivo experience with lithotripsy targeted away from bone at the kidney. T M From the findings in this experiment, lithotripsy has no beneficial effect on facilitating the removal of secure u n c e m e n t e d porous ingrowth devices. In addition, a n y misdirected focus from lithotripsy should have no deleterious effect on a secure imp l a n t - b o n e interface. However, at high kilovolts cortical fracture can potentially occur with exposure to high-magnitude shock waves.
References 1. Bobyn JD, Pilliar RM, Cameron HU, Weatherly GC: The optimum pore size for the fixation of porous-surfaced metal implants by the ingrowth of bone. Clin Orthop 150:263, 1980
2. Chaussy CH, Schmiedt E, Jocham D et al: Extracorporeal shock wave lithotripsy. Basel, Karger, 1982 3. Clemow AJT, Weinstein AM, Klawitter JJ et al: Interface mechanics of porous titanium implants. J Biomed Mater Res 15:73, 1981 4. Cook SD, Thomas KA, Kay JF, Jarcho M: Hydroxyapatite-coated porous titanium for use as an orthopaedic biologic attachment system. Clin Orthop 230:303, 1988 5. Cook SD, Walsh KA, Haddad RJ: Interface mechanics and bone growth into porous Co-Cr-Mo alloy implants. Clin Orthop 193:271, 1985 6. Eisenberger F, Chaussy C, Wanner E: Extrakorporale Anwendung von hochenergetishen Stobwellen: ein neuer Aspeekt in der Behanlung des Hamsteinleidens. Aktuelle Urologie 8:3, 1977 7. Galante J, Rostoker W, Lueck R, Ray RD: Sintered fiber metal composites as a basis for attachment of implants to bone. J Bone Joint Surg 53A:101, 197I 8. Harrigan TP, Jasty M, Davies JP, Harris WH: Interface failure assessed using Push-Out tests: example: the cement prosthesis interface. Trans Orthop Res Soc 13:501, 1988 9. Hunter PT, Finlayson B, Hirko RJ et al: Measurement of shock~wave pressures used for lithotripsy. J Urol 136:733, 1986 10. Kandel LB, Harrison LH, McCullough DL: State of the art extracorporeal shock wave lithotripsy. Futura, New York, 1987 11. Karpman RR, Magee FP, Gruen TWS, Mobley T: The lithotriptor and its potential use in the revision of total hip arthroplasty. Orth Rev 16:81, 1987 12. Lavernia CJ, Yoshida G, Reindel E et al: Effects of warfarin on the ingrowth kinetics of porous coated devices. Trans Orthop Res Soc 13:312, 1988 13. May TC, Krause WR, Preslar AJ et al: Use of highenergy shock waves for bone cement removal. J Arthroplasty 5:19, 1990 14. McDonald DJ, Fitzgerald RH, Chao EYS: The enhancement of fixation of a porous-coated femoral component by autograft and allograft in the dog. J Bone Joint Surg 70A:728, 1988 15. Michaels EK, Fowler JE Jr: Inadvertent fracture of gallstones during extracorporeal shock wave lithotripsy. J Urol 13:1285, 1986 16. Riehle RA, Newman RC: Principles of extracorporeal shock wave lithotripsy. Churchill Livingstone, New York, 1987 17. Rivero DP, Skipor AK, Singh M e t al: Effect of disodium etidronate on bone ingrowth in a porous material. Clin Orthop 215:279, 1987 18. Stranne SK, Callaghan JJ, Cocks FH et al: Revision arthroplasty facilitated by extracorporeal shock wave lithotripsy: an evaluation of whole bone strength. Clin Orthop (in press)
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19. Stranne SK, Myers BS, Weinerth JL et al: Lithotripsy facilitated revision of cemented arthroplasty and whole bone strength. Trans Orthop Res Soc 15:465, 1990 20. Weinstein JN, Oster DM, Park JB et al: The effect of the extracorporeal shock wave lithotriptor on the
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bone-cement interface in dogs. Clin Orthop 235:261, 1988 21. Weinstein JN, Wroble RR, Loening S: Revision total joint arthroplasty facilitated by extracorporeal shock wave lithotripsy: a case report. Iowa Orthop J 6:121, 1986
Steven K. S t r a n n e ,
MD,
Christian John
John
J. C a l l a g h a n ,
S. F u l g h u m ,
L. W e i n e r t h ,
MD, MD,
MD, Thomas
Richard
R. G l i s s o n ,
and Anthony
M. Fyda, MD, BS,
V. S e a b e r
Abstract: The effect of extracorporeal shock wave lithotripsy on interfacial strength between prosthesis and bone in cementless arthroplasty was examined using a rabbit model. Paired femora, each implanted with fiber mesh porous coated titanium implants, were harvested from rabbits 15 weeks after implantation. In group I, one femur from each pair was exposed to lithotripsy treatment consisting of 2,000 shocks at 20 kV. In group II, one femur from each pair was exposed to 2,000 shocks at 26 kV. Contralateral femora from each pair served as controls in both groups. Mechanical pushout tests were conducted on the implants using a 1321 tnstron testing machine at a constant rate of 1 ram/minute. Shock waves generated at 20 kV were found to have no significant decrease on either the prosthesis/bone interfacial strength or energy to failure of cementless implants. Shock waves generated at 26 kV produced a mean 17.45% decrease in the prosthesis/bone interfacial strength, which approached statistical significance (P = .062), and a 7.84% mean decrease in the energy to failure (P = .268). However, in four of the seven group II specimens, cortical fractures occurred. These findings suggest that lithotripsy will not aid in the removal of uncemented porous coated devices and lithotripsy inadvertently focused at an uncemented device will not disrupt significantly the prosthesis-bone interface. K e y words: lithotripsy, uncemented, prosthesis, rabbit.
A n u m b e r of investigators h a v e evaluated the use of extracorporeal shock w a v e lithotripsy ~~.~3,~8-2~ to facilitate the r e m o v a l of c e m e n t that is m e c h a n i c a l l y i n t e r l o c k e d w i t h b o n e . The effectiveness of litho
tripsy to disrupt the c e m e n t - b o n e interface has b e e n attributed to the acoustic i m p e d a n c e difference bet w e e n b o n e a n d b o n e cement. 2°,2t At a n interface b e t w e e n t w o materials, the force m a g n i t u d e delivered by a shock w a v e increases p r o p o r t i o n a t e l y w i t h the difference in a c o u s t i c i m p e d a n c e b e t w e e n t h e t w o materials.2,10,x6 A l t h o u g h n o investigations of the potential for high e n e r g y s h o c k w a v e s to disrupt the p r o s t h e s i s - b o n e interface in c e m e n t l e s s a r t h r o p l a s t y h a v e b e e n p e r f o r m e d , the a c o u s t i c
From the Division of Orthopaedic Surgery, Duke University Medical Center, Durham, North Carolina. Supported by the Zimmer Corporation and the Piedmont Othopaedic Society. Reprint requests: John J. Callaghan, MD, Department of Orthopaedics, University of Iowa, Iowa City, IA 52242.
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The Journal of Arthroplasty Vol. 7 No. 2 June 1992
impedance difference between bone and metal is significantly greater than the difference between bone and p o l y m e t h y l m e t h a c r y l a t e . Hence, lithotripsy could theoretically aid in disruption of the prost h e s i s - b o n e interface for r e p l a c e m e n t of uncem e n t e d prostheses even m o r e effectively t h a n the disruption of the c e m e n t - b o n e interface in cemented replacements. In addition to the potential efficacy of lithotripsy to facilitate u n c e m e n t e d revision arthroplasty, the safety of lithotripsy treatment for kidney and gallbladder calculi in patients with u n c e m e n t e d prostheses has been questioned but not addressed. Areas well outside of the lithotripter's focus are subjected to significant shock wave energy, as evidenced by the report of an inadvertent gallstone fracture secondary to shock wave treatment aimed at a kidney stone, x5 Acetabular prostheses and spinal instrumentation m a y come within centimeters of kidneys, ureters, gallbladders, and other lithotripsy targets. Thus, the potential effect of clinical lithotripsy treatment on the fixation of u n c e m e n t e d prostheses requires definition. This investigation is designed to examine the effect of extracorporeal shock wave lithotripsy on the prost h e s i s - b o n e interface in a cementless porous ingrowth arthroplasty model.
Materials and Methods Forty-two implants were placed bilaterally in the distal femoral metaphyses of 21 New Zealand white rabbits weighing 4 . 0 - 6 . 0 kg. The cylindrical implants (Zimmer, Warsaw, IN) were covered circumferentially with commercially pure titanium mesh porous coating. Cylinder dimensions were 4.6 m m in diameter and 9.1 m m in length. Implantation and subsequent mechanical testing in this rabbit model were performed as described by Lavernia et al. ~2 A 2 cm lateral incision exposed the distal femoral metaphysis. In each femur, a 5 m m drill hole was centered 1 m m proximal to the insertion of the lateral collateral ligament. Drill holes were oriented through the lateral cortex toward the medial cortex. The implants were pressed into the drill holes and soft tissues were closed with suture. Implantation was conducted using sterile technique. Postoperatively, the rabbits were returned immediately to unrestricted weight-bearing activity. A 7 day course of prophylactic t r i m e t h o p r i m - s u l f a m e t h o x a z o l e was administered by intramuscular injection. Fifteen weeks after the surgical implantation procedure, radiographs of the hind limbs were acquired
and the rabbits were killed. Bilateral femora were harvested and stripped of all soft tissues, including periosteum. Two groups consisting of,eight pairs each w e r e designated for mechanical study and one femur from each pair was selected for lithotripsy treatment. In both groups, the contralateral m e m b e r of each pair served as a paired control and received n o shock wave exposure. Each f e m u r to receive lithotripsy treatment was suspended by a specially designed rig in the water bath of the Dornier HM 3 lithotripter used clinically in this hospital. Shock waves were oriented sequentially from four different directions (Fig. i) and were targeted o n the bone-prosthesis interface. Group I femora received a total of 2,000 shocks administered in four groups of 500 at 20kV. Group II femora received a total of 2,000 shocks in four groups of 500 at 26 kV. The contralateral control femur of each pair did not receive exposure to lithotripsy but was soaked in a water bath for an equivalent time period. An additional five pairs of implants were tested mechanically and neither side received lithotripsy treatment in order to validate side-to-side paired comparison in this model. All femora were prepared identically for pushout testing. Bone tissue obstructing implant ends was removed with a high-speed burr. T h r o u g h o u t preparation and testing, the specimens remained at r o o m temperature and were kept moist with saline-soaked gauze to maintain the fully hydrated state. All specimens were tested within 8 hours of sacrifice. In mechanical pushout tests, each cylindrical implant was precisely aligned with the axis of test of a 1321 Instron testing machine. The diaphysis of the bone was m o u n t e d in polymethylcrylate and the distal edge of the condyle was supported with a metal
Fig. 1. Orientation of the four groups of shock waves targeted on the implants.
ESWL and Prosthesis Interface
Fig. 2. A specimen prepared for pushout testing in the Instron testing machine. brace to eliminate bending (Fig. 2). Mechanical axial pushout tests were performed at a constant rate of 1 mm/minute. Force versus deflection data were obtained and stored digitally on computer hard disk. The data were calibrated and used to calculate maxim u m force and strain energy to failure. Force of pushout was defined as the m a x i m u m force achieved during the test. Energy capacity was defined as the integral of the force versus displacement plot from zero displacement to the point of m a x i m u m force. To eliminate the effect of interspecimen variability, paired specimens and paired statistical analyses (oneway ANOVA) were used. In addition to mechanical testing, scanning electron microscopy was used to examine the surrounding bone and the retrieved specimen following pushout testing in one pair from both experimental and control groups (four specimens).
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discarded. In another pair, which was discarded, the core of the plug separated from the porous coating during pushout testing, representing poor sintering of the coating to the plug. Hence, for this report there were seven successfully treated pairs in each group. In every case, failure occurred at the i m p l a n t - b o n e interface and was not attributable to failure of the titanium fiber coating. Side-to-side comparison in untreated paired implants to validate the p u s h o u t testing m e t h o d revealed no statistical differences between sides (P = .28 by paired t-test) in force and energy capacity required to pushout. The m e a n force required for mechanical pushout tests is expressed as a percentage difference from the contralateral control of each pair (Fig. 5). Group I femora, exposed to 2,000 shocks at 20 kV, revealed no significant decrease in pushout strength or energy to failure as compared to controls. Only one pair demonstrated a decrease of 10% or greater than control for either parameter measured. Group II femora, exposed to 2,000 shocks at 26 kV, revealed a reduction in m e a n mechanical pushout strength of 17.45 %, which approached statistical significance (P = .0625) and a reduction in energy to
Results In all cases, radiographic evaluation revealed no lucencies at the implant interface and was consistent with significant bone ingrowth (Fig. 3). Gross examination of harvested femora revealed all implants to be stable without sign of fibrous encapsulation, infection, or inflammatory reaction. Following lithotripsy treatment, all implants remained grossly stable. Four of the seven lithotripsytreated femora in group II revealed grossly evident cortical fractures following treatment. This consisted of 1 0 - 2 0 m m fissures extending from the plug to the bone shaft in three specimens and a 3 x 8 m m defect created in the shaft of the fourth (Fig. 4). One of the bones in one pair from each of the groups fractured at the mounting interface during mechanical testing. In both cases the entire pair was
Fig. 3. Radiograph of implants following 15 weeks of ingrowth revealing no sign of loosening or infection.
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Fig. 4. Cortical fracture (white arrow) was grossly evident in two group II specimens.
failure of 7.84% (P -= .268). Six of the seven pairs in this group displayed a 10% or greater reduction in both force and energy. Scanning electron microscopy (Fig. 6) demonstrated no surrounding bone microfractures. However, as documented, gross fractures did occur in some group II experimental specimens. Scanning electron microscopy demonstrated similar findings in the control and experimental group pullout specimens. Failure occurred at the surrounding bone interface with some bone still attached to the porous plugs (Fig. 6B) in all specimens.
Discussion The specific aims of this investigation were to evaluate the use of extracorporeal shock wave lithotripsy applied to facilitate porous coated cementless arthroplasty revision and to evaluate any adverse effects o n the fixation of cementless implants by lithotripsy treatment aimed at renal or gallstone targets, This evaluation of the bone-prosthesis interface following shock wave exposure was considered because of the reported success of the lithotripter to disrupt the c e m e n t - b o n e interface 1~'x3"2°'21 and because of
120 100 % of 80 Control 60 40 20 0
force
energy
force
energy
20kV 26kV Fig. 5. Mean and standard deviation of the pushout force and energy to failure for group I (input voltage of 20 kV) and group II (input voltage of 26 kV) expressed as a percentage of paired contralateral control (N = seven pairs in each group).
the larger acoustic i m p e d e n c e m i s m a t c h b e t w e e n metal and bone than between cement and bone. A linear relationship exists between input kilovoltage and the pressure created at the focus characterized by the e q u a t i o n pressure psi = 0.347 kV - 0.669. 9 This study employed shock waves created by input voltages of either 20 kV or 26 kV, representing both moderate and high shock wave magnitudes. For renal stone comminution, voltages between 18 and 24 kV are c o m m o n l y employed for 1,200-1,800 shocks. In previous studies of cemented arthroplasty models, input voltages of 20 kV 2° and 25 kV ~3 have been s h o w n to decrease c e m e n t - b o n e interfacial strength significantly. To have clinical significance, the changes in interfacial ( b o n e - p r o s t h e s i s in this case) strength produced by a lithotripter to facilitate revision arthroplasty or to loosen a clinically secure prosthesis (from a misdirected focus) must be large (40% or greater). Based on side-to-side comparisons in our paired untreated controls, the sensitivity displayed in this mechanical pushout model allows detection of changes of this magnitude. Several additional factors idealize the shock wave pressures achieved at the bone-prosthesis interface and serve to increase the sensitivity of this model to detect disruption of the bone-prosthesis interface. The absence of soft tissues and the relatively small dimensions of the rabbit femora result in less attenuation of the shock wave energy than in the h u m a n clinical condition. In addition, the overall size of the implant allowed the e n t i r e implant to be placed within the focus of the HM 3 Dornier lithotripter. The implant would never fall directly in the focus during the clinical treatment of renal or gallbladder calculi. In addition, placing the entire implant w i t h i n the focus idealizes the clinical condition for facilitation of revision arthroplasty. The factors in this m o d e l maximize the shock wave pressures achieved at the i m p l a n t - b o n e interface. Multiple investigators have evaluated p u s h o u t tests to access the bone-prosthesis interface in po-
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Fig. 6. Scanning electron microscopy revealed no microfractures in the surrounding bone (A) and a thin layer of bone attached to the porous surface of the retrieved specimens (B).
rous coated implants. L3-5,7, 24, ~7 Pushout testing provides an effective m e a n s of comparing the relative interracial strength w h e n g e o m e t r y and load regim e n s are identical. 8 Potential sources of error include alignment of the implant with axis of test, taper or irregularity of the porous coated implant, and variability in the quality a n d q u a n t i t y of i n g r o w t h in paired specimens. An attempt was m a d e to minimize these variables in this investigation by modifying a previously used jig,22 which accurately aligns the implant with the axis of test and uses specifically designed and m a n u f a c t u r e d cylindrical plugs (Zimmer, Warsaw, IN). In addition, the specimens were tested soon after sacrifice, were maintained fully hydrated, and were not frozen. The m e a n values, standard deviations, and standard errors of the m e a n found in the present study correlate well with those reported previously. ~,3-5,7,14, 2 7 Scanning electron microscopy was used both to validate b o n y ingrowth in this rabbit model and to examine specimens after testing. Significant differences in cortical or trabecular microfractures were not detected either within groups or between groups.
However, group II (26 kV) specimens revealed overt fractures in four of seven specimens. Failure in the experimental and control groups in p u s h o u t testing occurred b e t w e e n the surrounding b o n e and a thin layer of bone attached to the porous surface (Fig. 6). Based on this investigation, extracorporeal shock w a v e lithotripsy is not useful for facilitating the revision of u n c e m e n t e d implants. This model provided a best-case scenario for disrupting the p r o s t h e s i s - b o n e interface because of minimal shock w a v e attenuation and placement of the entire implant within the lithotripter focus. H o w e v e r , in n e i t h e r g r o u p w a s the m e a n reduction in p u s h o u t strength great e n o u g h to warrant use of this technology. In group I (20kV), only one of the seven pairs exhibited a strength reduction of greater than 10% of control. This is in contrast to a cemented arthroplasty model, for w h i c h a reduction of 63% in interfacial strength was reported by Weinstein using an input voltage of 20 kV. 2° In group II pairs (26 kV), all but one of the treated pairs showed a reduction in both strength and energy to failure. Statistically, a trend toward decreased p u s h o u t force to failure (P = .0620) was
178 The Journal of Arthroplasty Vol. 7 No. 2 June 1992 demonstrated. However, the m e a n magnitude of reduction was small and probably of minimal conseq u e n c e for the clinical revision of prostheses. Because of the cortical fractures generated at this shock wave magnitude level, increasing the input voltage is inappropriate. Acetabular prostheses and spinal instrumentation m a y come within centimeters of the focus during lithotripsy treatment of urologic or gallbladder calculi. Of interest is whether either cementless implants will loosen w h e n subjected to lithotripsy or the presence of the metal implant will predispose the bone to fracture. Although the lithotripter was focused directly o n the implant, 2,000 shocks at 20 kV showed no effect on the strength of the u n c e m e n t e d implant interface in this investigation. The strength reduction seen in group II (26 kV) specimens was both small and exaggerated in this model due to direct focus on the implant. Pressures generated by shock w a v e s are greatest at the center of focus, and the pressure magnitude falls by more than 80% at a position 1.5 cm from the axis of focus. 9'16 In this rabbit model, bone fractures were created by 26 kV shock waves in femora containing metallic implants. While the thin cortex of the rabbit provides a sensitive m e a s u r e of such damage, fracture of h u m a n bones (without implants) exposed to shock waves targeted directly o n the b o n e has been reported. Fracture through the cortex of a flesh cadaveric h u m a n femur resulting from a single shock wave (25 kV) targeted directly at the bone 6 and the disruption of trabeculae in vertebrae subjected to several such shocks has been reported. 6,m However, no observable fractures of ribs or other bones have been demonstrated t h r o u g h o u t the significant in vivo experience with lithotripsy targeted away from bone at the kidney. T M From the findings in this experiment, lithotripsy has no beneficial effect on facilitating the removal of secure u n c e m e n t e d porous ingrowth devices. In addition, a n y misdirected focus from lithotripsy should have no deleterious effect on a secure imp l a n t - b o n e interface. However, at high kilovolts cortical fracture can potentially occur with exposure to high-magnitude shock waves.
References 1. Bobyn JD, Pilliar RM, Cameron HU, Weatherly GC: The optimum pore size for the fixation of porous-surfaced metal implants by the ingrowth of bone. Clin Orthop 150:263, 1980
2. Chaussy CH, Schmiedt E, Jocham D et al: Extracorporeal shock wave lithotripsy. Basel, Karger, 1982 3. Clemow AJT, Weinstein AM, Klawitter JJ et al: Interface mechanics of porous titanium implants. J Biomed Mater Res 15:73, 1981 4. Cook SD, Thomas KA, Kay JF, Jarcho M: Hydroxyapatite-coated porous titanium for use as an orthopaedic biologic attachment system. Clin Orthop 230:303, 1988 5. Cook SD, Walsh KA, Haddad RJ: Interface mechanics and bone growth into porous Co-Cr-Mo alloy implants. Clin Orthop 193:271, 1985 6. Eisenberger F, Chaussy C, Wanner E: Extrakorporale Anwendung von hochenergetishen Stobwellen: ein neuer Aspeekt in der Behanlung des Hamsteinleidens. Aktuelle Urologie 8:3, 1977 7. Galante J, Rostoker W, Lueck R, Ray RD: Sintered fiber metal composites as a basis for attachment of implants to bone. J Bone Joint Surg 53A:101, 197I 8. Harrigan TP, Jasty M, Davies JP, Harris WH: Interface failure assessed using Push-Out tests: example: the cement prosthesis interface. Trans Orthop Res Soc 13:501, 1988 9. Hunter PT, Finlayson B, Hirko RJ et al: Measurement of shock~wave pressures used for lithotripsy. J Urol 136:733, 1986 10. Kandel LB, Harrison LH, McCullough DL: State of the art extracorporeal shock wave lithotripsy. Futura, New York, 1987 11. Karpman RR, Magee FP, Gruen TWS, Mobley T: The lithotriptor and its potential use in the revision of total hip arthroplasty. Orth Rev 16:81, 1987 12. Lavernia CJ, Yoshida G, Reindel E et al: Effects of warfarin on the ingrowth kinetics of porous coated devices. Trans Orthop Res Soc 13:312, 1988 13. May TC, Krause WR, Preslar AJ et al: Use of highenergy shock waves for bone cement removal. J Arthroplasty 5:19, 1990 14. McDonald DJ, Fitzgerald RH, Chao EYS: The enhancement of fixation of a porous-coated femoral component by autograft and allograft in the dog. J Bone Joint Surg 70A:728, 1988 15. Michaels EK, Fowler JE Jr: Inadvertent fracture of gallstones during extracorporeal shock wave lithotripsy. J Urol 13:1285, 1986 16. Riehle RA, Newman RC: Principles of extracorporeal shock wave lithotripsy. Churchill Livingstone, New York, 1987 17. Rivero DP, Skipor AK, Singh M e t al: Effect of disodium etidronate on bone ingrowth in a porous material. Clin Orthop 215:279, 1987 18. Stranne SK, Callaghan JJ, Cocks FH et al: Revision arthroplasty facilitated by extracorporeal shock wave lithotripsy: an evaluation of whole bone strength. Clin Orthop (in press)
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19. Stranne SK, Myers BS, Weinerth JL et al: Lithotripsy facilitated revision of cemented arthroplasty and whole bone strength. Trans Orthop Res Soc 15:465, 1990 20. Weinstein JN, Oster DM, Park JB et al: The effect of the extracorporeal shock wave lithotriptor on the
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bone-cement interface in dogs. Clin Orthop 235:261, 1988 21. Weinstein JN, Wroble RR, Loening S: Revision total joint arthroplasty facilitated by extracorporeal shock wave lithotripsy: a case report. Iowa Orthop J 6:121, 1986