D i s p l a c e m e n t of an U n c e m e n t e d A c e t a b u l a r C o m p o n e n t A f t e r D i s l o c a t i o n of a T o t a l H i p Prosthesis A Case Report Richard
T. L a u g h l i n ,
M D , * K e v i n L. S m i t h , M D , ~ a n d
Daniel M. Adair, MD*
Abstract" Review of the literature reveals few reports of complications encountered with noncemented acetabular components; most concern problems with screw or cup placement, component wear or migration, or disassembly of modular components. No reports involving the displacement of a noncemented acetabular component were found. This is a case report of a patient in whom a noncemented acetabular component was dislodged after the closed reduction of a dislocated total hip prosthesis 4.5 weeks after surgery. In light of this case, the authors believe these reductions should be performed under general anesthesia with fluoroscopic guidance. Care must be taken at surgery to ream sufficiently and obtain proper cup fit and position. Finally, the authors recommend bicortical screw fixation to provide maximum contact and rigid fixation in the early postoperative period. K e y w o r d s : hip arthroplasty, dislocation, prosthesis, complications, acetabulum.
Recently, a report of d i s p l a c e m e n t of a n o n c e m e n t e d femoral c o m p o n e n t was published. 3 Review of the literature reveals few reports of complications e n c o u n t e r e d w i t h n o n c e m e m e d a c e t a b u l a r components. Complications that have been reported include false a n e u r y s m of the c o m m o n femoral artery secondary to migration of a threaded acetabular c o m p o n e n t , 5 abdominal hip joint fistula, 9 late hemarthrosis, x2 occlusion of the c o m m o n femoral artery, ~3 external iliac arteriovenous fistula, 15 and hip socket w e a r due to c o m p o n e n t mismatch. ~o In addition, several articles concerning the disassembly of m o d u l a r c o m p o n e n t s have been reported, including
both the femoral 11'~6 and acetabular 8 c o m p o n e n t s . No reports have been found of dislocation or dislodgement of a n o n c e m e n t e d acetabular c o m p o n e n t . This is a case report of a p a t i e n t in w h o m a n o n c e m e n t e d acetabular c o m p o n e n t was dislodged after the closed reduction of a dislocation of the total hip prosthesis 4.5 weeks after surgery.
Case Report A 60-year-old m a n had severe, disabling osteoarthritis of the left hip (Fig. 1). A left total hip arthroplasty utilizing a posterior a p p r o a c h was performed under spinal anesthesia using Harris-Galante c o m p o n e n t s (Zimmer, Warsaw, IN). A 56 m m porous-coated acetabular c o m p o n e n t was inserted after reaming from 44 m m to 56 m m . It was secured with three screws, and an elevated posterior lip polyethylene liner was inserted. The femoral canal was t h e n
From the *Division of Orthopaedics and Rehabilitation, Southern Illinois University, School of Medicine, Springfield, Illinois, and the fDepartment of Orthopaedic Surgery, McLaren Regional Medical Center, Flint, Michigan. Reprint requests: Richard T. Laughlin, MD, Division of Orthopaedics and Rehabilitation, Southern Illinois University, School of Medicine, P.O. Box 19230, Springfield, IL 62794-9230.
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reamed to 13 mm, followed by the placement of a 13 m m porous-coated femoral component. A 28 m m head with a m e d i u m neck was used and the hip was felt to be stable with range of motion testing (Fig. 2). After an uneventful postoperative course, the patient was discharged and instructed to sleep with a pillow b e t w e e n his legs. Partial weight-bearing with a walker was allowed. Four and one-half weeks after surgery the patient apparently crossed his legs while sleeping in a recliner without a pillow in place between his legs. He awoke with severe left hip pain and was unable to bear weight on the left lower extremity. Roentgenograms revealed a posterior dislocation of the left total hip (Fig. 3). The patient was transferred to S o u t h e r n Illinois University, where closed reduction with intravenous sedation was attempted. Longitudinal traction, flexion, and rotation were applied to the left lower limb. This subsequently failed, and closed reduction was t h e n p e r f o r m e d successfully u n d e r general anesthesia (Fig. 4). The patient was placed in an abductor splint. The following day the patient's left lower extremity was noted to be internally rotated and shortFig. 2. Postoperative roentgenogram after primary left total hip arthroplasty with a small gap in the medial bone-implant interface noted in retrospect.
Fig. I. Preoperative roentgenogram of the left hip showing severe osteoarthritis.
ened in bed. Roentgenograms showed dislocation of the acetabular cup from the pelvis (Fig. 5). The patient was taken to surgery for open reduction and reimplantation of the acetabular c o m p o n e n t under general anesthesia, again utilizing a posterior approach. Minimal ingrowth into the dislocated acetabular c o m p o n e n t was noted and a large piece of bone was found attached to one of the screws. The acetabulum was further reamed to 58 mm, followed by the insertion of a 58 m m porous-coated acetabular component, which was secured with five screws. A posterior lip liner was inserted into the component. Upon taking the hip through ranges of m o t i o n it was felt to impinge anteriorly. Thus, a long neck with a 28 m m head was utilized (Fig. 6). This resulted in good stability with flexion to 90 °, abduction to 20 °, and internal rotation to 15 °. The postoperative course was without complication and the patient was discharged with instructions to maintain partial weightbearing with a walker. At the 5 m o n t h follow-up visit, the patient had minimal pain and was able to walk without problems, only occasionally requiring the assistance of a cane. Roentgenograms showed continued good position of the acetabular and femoral components.
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Fig. 3. Roentgenogram after posterior dislocation of the left total hip arthroplasty.
Fig. 4. Roentgenogram following closed reduction under general anesthesia showing apparent good position of the acetabular component.
Fig. 5. Roentgenogram the following day with displacement of the acetabular cup from the pelvis.
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Fig. 6. Postoperative roentgenogram after revising the acetabular cup and utilizing a long neck femoral head showing good position of the components.
Discussion Fixation of n o n c e m e n t e d total hip prostheses in the early p o s t o p e r a t i v e period w a s addressed by Friedman in his article reporting the case of a displaced femoral c o m p o n e n t approximately 4 weeks after surgery. 3 Our case is very similar in that the acetabular c o m p o n e n t was displaced 4.5 weeks after surgery after attempted closed reduction under intravenous sedation followed by successful closed reduction u n d e r general anesthesia. M a n y studies h a v e outlined the time f r a m e in which b o n y ingrowth occurs, and m u c h of this w o r k was done o n the femoral component. Engh et al. reported on the retrieval of a femoral stem at 6 weeks that s h o w e d evidence of b o n y ingrowth. 2 Bobyn et al. ~ also noted that m a x i m a l ingrowth occurs at 6 - 8 weeks. In addressing a c e t a b u l a r fixation, Galante describes several phases of b o n y ingrowth. 4 Initially, a nonspecific i n f l a m m a t o r y response occurs that lasts a few days. This is replaced by osteoprogenitor mesenchyme, which evolves into w o v e n b o n e within the
porous structure. This becomes apparent at approximately 1 week. This reparative stage is variable and is followed by a stage of appositional b o n e formation producing true lamellar bone. The remodelling of the lamellar bone parallels the weight-bearing stresses. Galante does note that the majority of the studies on ingrowth have involved the femoral c o m p o n e n t . Since m a x i m u m i n g r o w t h does n o t o c c u r until 6 - 8 weeks after surgery, certain measures m u s t be taken to protect the implant. This begins with the provisional fixation. G a l a n t e ' s r e c o m m e n d a t i o n s 4 include the presence of a tight press fit in w h i c h there are no gaps greater t h a n 0.5 m m in the b o n e - i m p l a n t interface, and the insertion of a prosthesis corresponding in size to the last reamer, secured with two or three screws. The i m p o r t a n c e of e n g a g i n g the inner wall of the pelvis with the screws was not addressed. In his canine studies, Harris 6 m e n t i o n e d the i m p o r t a n c e of bicortical fixation in p r o v i d i n g compression between the c o m p o n e n t and the acetabular bed, thus maximizing the contact and providing rigid fixation. To avoid neurovascular complications secondary to acetabular screw placement, t w o recent articles 7"14 r e c o m m e n d placing screws generally in the posterior quadrants of the a c e t a b u l u m to reduce the chance of problems. Bicortical screw fixation of the acetabular c o m p o nent is not universally accepted. The emphasis seems to be on obtaining a press fit. Some r e c o m m e n d using a cup that is 2 m m larger than the last r e a m e r to obtain the press fit. If this is done, screw fixation m a y not be necessary. Friedman 3 thought that a lever-type force applied to the p o s t e r o s u p e r i o r edge of the a c e t a b u l a r cup caused the displacement of the femoral prosthesis. While this same type of force m a y have contributed to the displacement of the acetabular c o m p o n e n t in our patient, we believe it m a y not h a v e been the only factor involved. In retrospect, we believe that there was a gap in the medial b o n e - i m p l a n t interface that m a y have also played a part in the failure of the acetabular component. Also, the cup should be medialized to the teardrop on the AP pelvis radiograph, which has not done in this case (Fig. 2). Finally, a lack of complete superolateral cup coverage, as well as a greater t h a n desired abduction angle of the cup, should be noted. Cup coverage w o u l d h a v e b e e n improved with better medialization. The m o s t stable position for cup placement has b e e n reported to be 300-50 ° of abduction and 5°-25 ° of anteversion. In the surgical technique description for the system used in this report, the r e c o m m e n d e d ideal p o s i t i o n is listed as 30 ° of abduction and 20 ° of forward flexion. The abduction angle of this patient's cup was larger than desired. Thus, while the closed reduction of the
Displacement of an Uncemented Acetabular Component d i s l o c a t e d t o t a l h i p p r o s t h e s i s m a y w e l l h a v e particip a t e d in t h e d i s l o d g m e n t o f t h e a c e t a b u l a r c o m p o n e n t , t h e p o s i t i o n of t h e c o m p o n e n t w i t h i n t h e b o n y pelvis m a y h a v e p r e s e n t e d a n u n d e r l y i n g instability, l e a v i n g it v u l n e r a b l e to d i s p l a c e m e n t . I n light o f this case, w e c o n c u r w i t h the r e c o m m e n d a t i o n of F r i e d m a n c o n c e r n i n g t h e m a n a g e m e n t of a d i s l o c a t e d u n c e m e n t e d total h i p p r o s t h e s i s in t h e e a r l y p o s t o p e r a t i v e p e r i o d . He r e c o m m e n d e d d o i n g t h e r e d u c t i o n u n d e r g e n e r a l a n e s t h e s i a to o b t a i n maximum muscle relaxation and using fluoroscopy to e n s u r e t h a t t h e f e m o r a l h e a d doe~~ n o t c a t c h o n t h e a c e t a b u l a r r i m a n d a l l o w d i s p l a c e m e n t . Special care m u s t be t a k e n in r e a m i n g sufficiently to assure p r o p e r fit o f t h e c u p in a d e f o r m e d , sclerotic a c e t a b u l u m . The i m p o r t a n c e of m e d i a l i z i n g t h e cup a n d obt a i n i n g a t i g h t press fit is illustrated in this case as well. This m u s t b e d o n e r e g a r d l e s s of w h e t h e r screw f i x a t i o n is used. T h o u g h stability is d e t e r m i n e d b y p o s i t i o n o f t h e c o m p o n e n t s , w e r e c o m m e n d screw f i x a t i o n to p r e v e n t d i s p l a c e m e n t u n t i l i n g r o w t h occurs.
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 I50:263, 1980 2. Engh CA, Bobyn JD, Glassman AH: Porous-coated hip replacement: the factors governing bone ingrowth, stress shielding, and clinical results. J Bone Joint Surg 69B:45, 1987 3. Friedman RJ: Displacement of an uncemented femoral c o m p o n e n t after dislocation of a to~al hip replacement: a case report. J Bone Joint Surg 71A: 1406, 1989 4. Galante JO: Acetabular fixation: clinical problems and possible solutions, p. i87. In The hip: proceedings of the Thirteenth Open Scientific Meeting of the Hip Society. CV Mosby, St. Louis, 1985
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5. Giacchetto J, Gallagher J J: False aneurysm of the comm o n femoral artery secondary to migration of a threaded acetabular component: a case report and review of the literature. Clin Orthop 231:91, 1988 6. Harris WH, Jasty M: Bone i n g r o w t h into porous coated canine acetabular replacements: the effect of pore size, apposition and dislocation, p. 214. In The hip: proceedings of the Thirteenth Open Scientific Meeting of the Hip Society. CV Mosby, St. Louis, 1985 7. Keating EM, Ritter MA, Faris PM: Structures at risk from medially placed acetabular screws. J Bone Joint Surg 72A:509, 1990 8. Kitziger K J, Delee JC, Evans JA: Disassembly of a modular acetabular component of a total hip replacement arthroplasty: a case report. J Bone Joint Surg 72A:62 I, 1990 9. Korovesis P, Siablis D, Salonikidis P, Sdougos G: Abdominal hip joint fistula. Complicated revision of total hip arthroplasty for false aneurysm of external iliac artery: a case report. Clin Orthop 231:71, 1988 10. L]ung P, Lidgren L, Rydholm U: Hip socket wear due to component mismatch: a case report. Acta Orthop Scand 60:223, 1989 11. Pellicci PM, Haas SB: Disassembly of a modular femoral component during closed reduction of the dislocated femoral component: a case report. J Bone Joint Surg 72A:619, 1990 12. Roberson JR: Late hemarthrosis from a threaded acetabular component. J Arthroplasty 3:61, 1988 13. Rutsaert R, Van Schil P, Martens C et al: Occlusion of the left common femoral artery after total hip replacement: report of a case and review of the literature. J Cardiovasc Surg 29:216, 1988 14. Wasielewski RC, Cooperstein LA, Kruger MP, Rubash HE: Acetabular anatomy and the transacetabular fixation of screws in total hip arthroplasty. J Bone Joint Surg 72A:501, 1990 15. Woolson ST, Maloney W J, Tanner JB: External iliac arteriovenous fistula following total hip arthroplasty: a case report. J Arthroplasty 4:281, i985 I6. Woolson ST, Pottorff GT: Disassembly of a modular femoral prosthesis after dislocation of the femoral component: a case report. J Bone Joint Surg 72A:624, 1990
T. L a u g h l i n ,
M D , * K e v i n L. S m i t h , M D , ~ a n d
Daniel M. Adair, MD*
Abstract" Review of the literature reveals few reports of complications encountered with noncemented acetabular components; most concern problems with screw or cup placement, component wear or migration, or disassembly of modular components. No reports involving the displacement of a noncemented acetabular component were found. This is a case report of a patient in whom a noncemented acetabular component was dislodged after the closed reduction of a dislocated total hip prosthesis 4.5 weeks after surgery. In light of this case, the authors believe these reductions should be performed under general anesthesia with fluoroscopic guidance. Care must be taken at surgery to ream sufficiently and obtain proper cup fit and position. Finally, the authors recommend bicortical screw fixation to provide maximum contact and rigid fixation in the early postoperative period. K e y w o r d s : hip arthroplasty, dislocation, prosthesis, complications, acetabulum.
Recently, a report of d i s p l a c e m e n t of a n o n c e m e n t e d femoral c o m p o n e n t was published. 3 Review of the literature reveals few reports of complications e n c o u n t e r e d w i t h n o n c e m e m e d a c e t a b u l a r components. Complications that have been reported include false a n e u r y s m of the c o m m o n femoral artery secondary to migration of a threaded acetabular c o m p o n e n t , 5 abdominal hip joint fistula, 9 late hemarthrosis, x2 occlusion of the c o m m o n femoral artery, ~3 external iliac arteriovenous fistula, 15 and hip socket w e a r due to c o m p o n e n t mismatch. ~o In addition, several articles concerning the disassembly of m o d u l a r c o m p o n e n t s have been reported, including
both the femoral 11'~6 and acetabular 8 c o m p o n e n t s . No reports have been found of dislocation or dislodgement of a n o n c e m e n t e d acetabular c o m p o n e n t . This is a case report of a p a t i e n t in w h o m a n o n c e m e n t e d acetabular c o m p o n e n t was dislodged after the closed reduction of a dislocation of the total hip prosthesis 4.5 weeks after surgery.
Case Report A 60-year-old m a n had severe, disabling osteoarthritis of the left hip (Fig. 1). A left total hip arthroplasty utilizing a posterior a p p r o a c h was performed under spinal anesthesia using Harris-Galante c o m p o n e n t s (Zimmer, Warsaw, IN). A 56 m m porous-coated acetabular c o m p o n e n t was inserted after reaming from 44 m m to 56 m m . It was secured with three screws, and an elevated posterior lip polyethylene liner was inserted. The femoral canal was t h e n
From the *Division of Orthopaedics and Rehabilitation, Southern Illinois University, School of Medicine, Springfield, Illinois, and the fDepartment of Orthopaedic Surgery, McLaren Regional Medical Center, Flint, Michigan. Reprint requests: Richard T. Laughlin, MD, Division of Orthopaedics and Rehabilitation, Southern Illinois University, School of Medicine, P.O. Box 19230, Springfield, IL 62794-9230.
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reamed to 13 mm, followed by the placement of a 13 m m porous-coated femoral component. A 28 m m head with a m e d i u m neck was used and the hip was felt to be stable with range of motion testing (Fig. 2). After an uneventful postoperative course, the patient was discharged and instructed to sleep with a pillow b e t w e e n his legs. Partial weight-bearing with a walker was allowed. Four and one-half weeks after surgery the patient apparently crossed his legs while sleeping in a recliner without a pillow in place between his legs. He awoke with severe left hip pain and was unable to bear weight on the left lower extremity. Roentgenograms revealed a posterior dislocation of the left total hip (Fig. 3). The patient was transferred to S o u t h e r n Illinois University, where closed reduction with intravenous sedation was attempted. Longitudinal traction, flexion, and rotation were applied to the left lower limb. This subsequently failed, and closed reduction was t h e n p e r f o r m e d successfully u n d e r general anesthesia (Fig. 4). The patient was placed in an abductor splint. The following day the patient's left lower extremity was noted to be internally rotated and shortFig. 2. Postoperative roentgenogram after primary left total hip arthroplasty with a small gap in the medial bone-implant interface noted in retrospect.
Fig. I. Preoperative roentgenogram of the left hip showing severe osteoarthritis.
ened in bed. Roentgenograms showed dislocation of the acetabular cup from the pelvis (Fig. 5). The patient was taken to surgery for open reduction and reimplantation of the acetabular c o m p o n e n t under general anesthesia, again utilizing a posterior approach. Minimal ingrowth into the dislocated acetabular c o m p o n e n t was noted and a large piece of bone was found attached to one of the screws. The acetabulum was further reamed to 58 mm, followed by the insertion of a 58 m m porous-coated acetabular component, which was secured with five screws. A posterior lip liner was inserted into the component. Upon taking the hip through ranges of m o t i o n it was felt to impinge anteriorly. Thus, a long neck with a 28 m m head was utilized (Fig. 6). This resulted in good stability with flexion to 90 °, abduction to 20 °, and internal rotation to 15 °. The postoperative course was without complication and the patient was discharged with instructions to maintain partial weightbearing with a walker. At the 5 m o n t h follow-up visit, the patient had minimal pain and was able to walk without problems, only occasionally requiring the assistance of a cane. Roentgenograms showed continued good position of the acetabular and femoral components.
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Fig. 3. Roentgenogram after posterior dislocation of the left total hip arthroplasty.
Fig. 4. Roentgenogram following closed reduction under general anesthesia showing apparent good position of the acetabular component.
Fig. 5. Roentgenogram the following day with displacement of the acetabular cup from the pelvis.
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Fig. 6. Postoperative roentgenogram after revising the acetabular cup and utilizing a long neck femoral head showing good position of the components.
Discussion Fixation of n o n c e m e n t e d total hip prostheses in the early p o s t o p e r a t i v e period w a s addressed by Friedman in his article reporting the case of a displaced femoral c o m p o n e n t approximately 4 weeks after surgery. 3 Our case is very similar in that the acetabular c o m p o n e n t was displaced 4.5 weeks after surgery after attempted closed reduction under intravenous sedation followed by successful closed reduction u n d e r general anesthesia. M a n y studies h a v e outlined the time f r a m e in which b o n y ingrowth occurs, and m u c h of this w o r k was done o n the femoral component. Engh et al. reported on the retrieval of a femoral stem at 6 weeks that s h o w e d evidence of b o n y ingrowth. 2 Bobyn et al. ~ also noted that m a x i m a l ingrowth occurs at 6 - 8 weeks. In addressing a c e t a b u l a r fixation, Galante describes several phases of b o n y ingrowth. 4 Initially, a nonspecific i n f l a m m a t o r y response occurs that lasts a few days. This is replaced by osteoprogenitor mesenchyme, which evolves into w o v e n b o n e within the
porous structure. This becomes apparent at approximately 1 week. This reparative stage is variable and is followed by a stage of appositional b o n e formation producing true lamellar bone. The remodelling of the lamellar bone parallels the weight-bearing stresses. Galante does note that the majority of the studies on ingrowth have involved the femoral c o m p o n e n t . Since m a x i m u m i n g r o w t h does n o t o c c u r until 6 - 8 weeks after surgery, certain measures m u s t be taken to protect the implant. This begins with the provisional fixation. G a l a n t e ' s r e c o m m e n d a t i o n s 4 include the presence of a tight press fit in w h i c h there are no gaps greater t h a n 0.5 m m in the b o n e - i m p l a n t interface, and the insertion of a prosthesis corresponding in size to the last reamer, secured with two or three screws. The i m p o r t a n c e of e n g a g i n g the inner wall of the pelvis with the screws was not addressed. In his canine studies, Harris 6 m e n t i o n e d the i m p o r t a n c e of bicortical fixation in p r o v i d i n g compression between the c o m p o n e n t and the acetabular bed, thus maximizing the contact and providing rigid fixation. To avoid neurovascular complications secondary to acetabular screw placement, t w o recent articles 7"14 r e c o m m e n d placing screws generally in the posterior quadrants of the a c e t a b u l u m to reduce the chance of problems. Bicortical screw fixation of the acetabular c o m p o nent is not universally accepted. The emphasis seems to be on obtaining a press fit. Some r e c o m m e n d using a cup that is 2 m m larger than the last r e a m e r to obtain the press fit. If this is done, screw fixation m a y not be necessary. Friedman 3 thought that a lever-type force applied to the p o s t e r o s u p e r i o r edge of the a c e t a b u l a r cup caused the displacement of the femoral prosthesis. While this same type of force m a y have contributed to the displacement of the acetabular c o m p o n e n t in our patient, we believe it m a y not h a v e been the only factor involved. In retrospect, we believe that there was a gap in the medial b o n e - i m p l a n t interface that m a y have also played a part in the failure of the acetabular component. Also, the cup should be medialized to the teardrop on the AP pelvis radiograph, which has not done in this case (Fig. 2). Finally, a lack of complete superolateral cup coverage, as well as a greater t h a n desired abduction angle of the cup, should be noted. Cup coverage w o u l d h a v e b e e n improved with better medialization. The m o s t stable position for cup placement has b e e n reported to be 300-50 ° of abduction and 5°-25 ° of anteversion. In the surgical technique description for the system used in this report, the r e c o m m e n d e d ideal p o s i t i o n is listed as 30 ° of abduction and 20 ° of forward flexion. The abduction angle of this patient's cup was larger than desired. Thus, while the closed reduction of the
Displacement of an Uncemented Acetabular Component d i s l o c a t e d t o t a l h i p p r o s t h e s i s m a y w e l l h a v e particip a t e d in t h e d i s l o d g m e n t o f t h e a c e t a b u l a r c o m p o n e n t , t h e p o s i t i o n of t h e c o m p o n e n t w i t h i n t h e b o n y pelvis m a y h a v e p r e s e n t e d a n u n d e r l y i n g instability, l e a v i n g it v u l n e r a b l e to d i s p l a c e m e n t . I n light o f this case, w e c o n c u r w i t h the r e c o m m e n d a t i o n of F r i e d m a n c o n c e r n i n g t h e m a n a g e m e n t of a d i s l o c a t e d u n c e m e n t e d total h i p p r o s t h e s i s in t h e e a r l y p o s t o p e r a t i v e p e r i o d . He r e c o m m e n d e d d o i n g t h e r e d u c t i o n u n d e r g e n e r a l a n e s t h e s i a to o b t a i n maximum muscle relaxation and using fluoroscopy to e n s u r e t h a t t h e f e m o r a l h e a d doe~~ n o t c a t c h o n t h e a c e t a b u l a r r i m a n d a l l o w d i s p l a c e m e n t . Special care m u s t be t a k e n in r e a m i n g sufficiently to assure p r o p e r fit o f t h e c u p in a d e f o r m e d , sclerotic a c e t a b u l u m . The i m p o r t a n c e of m e d i a l i z i n g t h e cup a n d obt a i n i n g a t i g h t press fit is illustrated in this case as well. This m u s t b e d o n e r e g a r d l e s s of w h e t h e r screw f i x a t i o n is used. T h o u g h stability is d e t e r m i n e d b y p o s i t i o n o f t h e c o m p o n e n t s , w e r e c o m m e n d screw f i x a t i o n to p r e v e n t d i s p l a c e m e n t u n t i l i n g r o w t h occurs.
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 I50:263, 1980 2. Engh CA, Bobyn JD, Glassman AH: Porous-coated hip replacement: the factors governing bone ingrowth, stress shielding, and clinical results. J Bone Joint Surg 69B:45, 1987 3. Friedman RJ: Displacement of an uncemented femoral c o m p o n e n t after dislocation of a to~al hip replacement: a case report. J Bone Joint Surg 71A: 1406, 1989 4. Galante JO: Acetabular fixation: clinical problems and possible solutions, p. i87. In The hip: proceedings of the Thirteenth Open Scientific Meeting of the Hip Society. CV Mosby, St. Louis, 1985
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