An oblong revision cup for large acetabular defects

An oblong revision cup for large acetabular defects

The Journal of Arthroplasty Vol. 13 No. 5 1998 An Oblong Revision Cup for Large Acetabular Defects Design Rationale and Two- to Seven-Year Follow-up ...

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The Journal of Arthroplasty Vol. 13 No. 5 1998

An Oblong Revision Cup for Large Acetabular Defects Design Rationale and Two- to Seven-Year Follow-up G e o r g K6ster, MD, H a n s - G e o r g Willert, MD, H o r s t - P e t e r K6hler, MD, and Karen D6pkens

Abstract: Loosening and migration of acetabular components often lead to extensive bony defects with an elongated, oval acetabular cavity. In these cases standard implants will not reestablish and maintain sufficient stability without leaving bone defects or using massive bone grafts or excess cement and additional metal rings or shells, disadvantages that are overcome by using an oblong revision cup without cement. The titanium shell is available in different sizes, is screwed to the autochthonous acetabular bone and houses an oblong polyethylene inlay, designed to reestablish the normal anatomic hip center. Of 109 consecutive revision cups, 102, implanted for American Academy of Orthopaedic Surgeons (AAOS) defects types I-IV, were followed up clinically and radiologically for 2 to 7 years (mean, 3.6 years). Primary stability was achieved in all cases. In 40% no bone grafting was necessary at all. The radiological follow-up revealed good remodeling of the surrounding bone and osseointegration of the implants. Zonal radiolucent lines, always smaller than 2 mm, were seen in 18 cases, only once completely and in only 5 cases partially progressing. Six cups migrated slightly (-<2 ram), two moderately (3-5 mm), all without clinical symptoms, and two more than 5 mm. Migration and radiolucencies were mainly seen in patients with allografts and major defects, which indicates that bone ingrowth appears more unlikely in such cases. Few asymptomatic cases showed zonal sclerotic lines. There were two aseptic loosenings, one in a case with pelvic discontinuity, the other in a patient with severe rheumatoid arthritis following two previous revisions. Survivorship analysis based on implant removal because of aseptic loosening as the endpoint shows a cumulative success rate of 98.1% at 8 years. K e y w o r d s : hip, revision arthroplasty, acetabular defects, oblong cup, cementless fixation, bone grafting.

Defects in the acetabular bed resulting from aseptic loosening of socket implants are caused by bone resorption due to implant migration and tissue reactions to particles of abraded polyethylene, disin-

tegrated b o n e c e m e n t or c o n s t i t u e n t s of o t h e r i m p l a n t s [1,2]. Various defect classifications h a v e b e e n p r o p o s e d to describe t h e typical forms of defects a n d to facilitate a n exact a s s e s s m e n t of the e x t e n t of osteolyses [3-5]. Often the s u p e r i o r or s u p e r o l a t e r a l m i g r a t i o n of the c o m p o n e n t causes a n oval a c e t a b u lar defect [6]. A c o m b i n e d s u p e r i o r a n d p o s t e r i o r segmental cavitary deficiency resulting in an oblong defect is a c o m m o n i n d i c a t i o n for revision. D e p e n d i n g o n the e x t e n t of t h e osteolytic defect, revision is c o n s i d e r a b l y m o r e difficult or e v e n i m p o s -

From the Department of Orthopaedic Surgery, Ceorg-AugustUniversit~it G~ttingen, Gbttingen, German},. Reprint requests: Georg K~ster, MD, Department of Orthopaedic Surgery, Georg-August-Universit~t G6ttingen, Robert-KochStral~e 40, D-37075 G6[tingen, Germany. Copyright © 1998 by Churchill Livingstone® 0883-5403/1305-001253.00/0

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sible with a conventional implant, especially if, as is often the case, the a c e t a b u l u m exhibits oval elongation and its longitudinal diameter is greatly in excess of its transverse diameter. If such a typical oblong defect is revised with a standard round, conical, or spherical implant, it has either to be widened in its dorsoventral diameter, which m e a n s additional bone loss endangering the stability of the pelvic ring, or it has to be filled with large a m o u n t s of allograft or b o n e cement. Sometimes it is necessary to use additional technical appliances like supporting rings, wire meshes, and fixation screws. Large defects, pelvic instability, and discontinuity require extensive additional supports with osteosynthesis plates. The disadvantages of the applying standard concepts for reconstruction in large acetabular defects have been widely discussed, especially concerning the use of c e m e n t and allografts [7-13]. The high complication rate following revision surgery [ 14-16] (especially multiple interventions [ 17]) emphasizes the need for better l o n g - t e r m solutions. However, p r i m a r y stability is surely the indispensible prerequisite for the osseous integration and thus for the lasting stability of an implant. These considerations and practical experience with customm a d e implants, provided the b a c k g r o u n d for the d e v e l o p m e n t of a n e w revision implant. The objective was to achieve p r i m a r y stable n o n c e m e n t e d anchorage w i t h o u t using extensive bone allografting and thus to optimize osseous integration. The implant should adapt to the preexisting defect in size and shape w i t h o u t sacrificing residual b o n e and should be supported on the available residual bone, so that only remaining defects h a v e to be filled with transplant bone. The result was a longitudinal oblong revision (LOR) cup designed for serial production.

(Fig. 1A). There is an oblong w i n d o w in the b o t t o m of the shell t h r o u g h w h i c h b o n e transplants can be packed behind. A flap on the caudal and three spring lamellae on the opposite cranial circumference of the shell secure the p o l y e t h y l e n e inlay after it has snapped into place (Fig. 1A). The inlay, produced from ultrahigh molecular weight polyethylene SULENE®-PE, is h o o d e d on the roof of the articular surface. Toward the pole, its level surface is covered with a coarse-finish pure titanium plate to p r e v e n t direct contact b e t w e e n p o l y e t h y l e n e and

Material and Methods Components The LOR cup consists of a titanium shell and a p o l y e t h y l e n e inlay. C o u n t e r s u n k t i t a n i u m screws are provided for fixation. The shell is produced from p u r e titanium (PROTASUL®-Ti) by a special deep drawing process [18]. Its geometric shape in the transverse axis looks like a h e m i s p h e r e w i t h o u t a pole segment. In the longitudinal axis, the h e m i sphere is elongated by 6 or 12 m m . Two rows of bore holes are arranged parallel to the outer rim of the shell to take the c o u n t e r s u n k screws. The screws, m a d e of a titanium alloy, are used for p r i m a r y fixation of shell to the pelvic bone. The thread type corresponds to standard AO (ASIF) spongiosa screws

Fig. 1. (A) Oblong titanium shell made of pure titanium with a coarse-blasted surface. Through the oblong window in the bottom bone transplants can be packed behind even after anchorage. The bore holes can take up the screws for additional fixation in the autochthonous bone. (B) The inlay produced from ultrahigh molecular weight polyethylene corresponds to the oblong internal form of the shell. At the bottom it is covered with a coarse-finish pure titanium plate in order to prevent direct contact between polyethylene and bone.

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b o n e (Fig. 1B). Three flat recesses are located in the cranial circumference of the PE inlay into which the spring lamellae of the titanium shell lock into place. On the caudal side the polyliner is held in position by a rigid rim. In the 6 - m m longitudinally elongated sockets, the articular surface is centrally positioned. In the m o r e elongated sockets (by 12 m m ) , the articular surface is centrally or 6 m m caudally displaced, so that both the reconstruction of the articulation center and soft-tissue tension can be t a k e n into account. The range of implants comprises a total of 12 titanium shells. Their transverse diameters correspond to the standard external dimensions of spherical sockets and thus also to the appropriate r e a m e r sizes in 4 m m steps f r o m 52 to 72. For each of these transverse diameters there are t w o shells available, one elongated by 6 and one by 12 m m longitudinally. Two shell sizes, 52 and 56, 60 and 64, 68 and 72, respectively, h a v e identical internal dimensions, so that the same PE inlays can be used, resulting in a total of 9 p o l y e t h y l e n e inlays.

a n g i o g r a p h y before surgery. Ideally, at least three screws should be placed cranially into the ilium, while additional stabilization can be achieved by placing further screws in the pubis and ischium. The appropriate p o l y e t h y l e n e inlay is inserted using the snap-fit m e c h a n i s m .

Surgical Technique

Patients and Evaluation

The b o n y a c e t a b u l u m is prepared using standard, spherical socket r e a m e r s by which the entrance of the a c e t a b u l u m can be widened, and obstructing ledges or protrusions of the b o n y bed can be removed. Small cavitary defects can be s m o o t h e d or filled with b o n e chips. Widening of the a c e t a b u l u m in the anteroposterior d i a m e t e r should be avoided. Trial implants are used to select the best-suited titanium shell. The titanium shell is inserted as a slightly oversized press-fit and is a n c h o r e d with screws. It is m a n d a t o r y that these screws should only be positioned within the bone. In cases of severe protrusion, it m a y be useful to p e r f o r m

B e t w e e n 1989 and 1994, 109 LOR cups h a v e b e e n implanted in 104 patients (78 female, 26 male) with an average age of 67.4 years (range, 4 3 - 8 9 years) at the time of surgery. Clinical and radiological follow-up was p e r f o r m e d in 97 patients with 102 implants over a period of 2 and 7 years (mean, 3.6 years), 4 patients died because of unrelated disease, and 3 w e r e lost to follow-up (Table 1); 98 patients had had a previous total hip r e p l a c e m e n t (Table 2). Revision surgery was p e r f o r m e d for the first time in 74 cases, for the second time in 15 cases, for the third time in 2 cases, and for the fourth time in 1 case. The indications for implantation of the revi-

Table

l 2 3 4 5 6 7 8

Total (New) 109 109 74 48 28 16 9 5

The LOR cup has b e e n specially designed for the reoperation of dislocated and loose ( c e m e n t e d and n o n c e m e n t e d ) socket implants in which m o r e or less p r o n o u n c e d osteolysis has occurred in the pelvic bone, thus leading to cranial a n d / o r caudal b o n y defects in the acetabulum. It is also indicated for p r i m a r y implantation in the case of an elongated or otherwise destroyed a c e t a b u l u m in which the implantation of a conventional socket is unreliable or no longer possible. Such indications can arise in secondary arthrosis due to dysplasia or Perthes' disease as well as in certain cases of r h e u m a t o i d arthritis.

1. N u m b e r o f C a s e s E v a l u a t e d at D i f f e r e n t F o l l o w - u p I n t e r v a l s

Cases F o l l o w - u p (year)

Indications

(0) (35) (26) (20) (12) (7) (4) (5)

W i t h d r a w n Cases

Clinical a n d

Deceased

Lost to F o l l o w - u p

Revisions

Radiological E x a m i n a t i o n

2 2 0 0 0 0 0 0

0 1 2 0 0 0 0 0

1 1 1" 0 0 0 0 0

106 102 71 48 28 16 9 5

From the 109 implanted devices, 7 were w i t h d r a w n from the series because the patients deceased or were lost to follow up leaving a total of 102 implants. Cups revised because of aseptic or septic loosening and cases where patients deceased or were lost to follow-up are listed here at their last follow-up before being withdrawn. *Septic loosening.

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The Journal of Arthroplasty Vol. 13 No. 5 August 1998 Table 2. Indications for Procedures and Patient Characteristics

Indication for Primary THR

n

Indication for LOR-Cup Implantation (n = 102)

n

Age (years)

n

Primary osteoarthritis Dysplasia Rheumatoid arthritis Avascular necrosis Posttraumatic arthritis Septic arthritis No primary THR

75 12 8 1 1 1 4

Aseptic loosening (cemented cup) Septic loosening (cemented cup) Aseptic loosening (noncemented cup) Septic loosening (noncemented cup) Dysplasia, previous Chiari osteotomy Severe protrusion, renal osteopathy Rheumatoid arthritis

30 7 61 0 1 1 2

<40 40-50 51-60 61-70 71-80 81-90 >91

0 4 11 36 42 9 0

Concomitant Disease/Risk Factor

n

Osteoarthritis contralateral hip Previous operation contralateral hip Implant loosening contralateral hip Cardiovascular disease Neurological disease Diabetes

13 55 3 61 11 7

THR, lotal hip replacement.

sion cup are listed in Table 2. In 53 cases only the cup had to be revised, but in 45 cases the stern had to be exchanged as well. The defects treated comprise all degrees of severity, including those with pelvic discontinuity, although the implant was not used in the case of minimal defects and arthrodesis. All defects were classified during surgery according to the American Academy of Orthopaedic Surgeons classification [5]. There were 22 (21.6%) segmental deficiencies (type I), 43 (42.2%) cavitary defects (type II), 33 (32.4%) combined defects (type III), and 4 cases (4.9%) of pelvic discontinuity (type IV). Among the segmental defects there were 9 peripheral, 2 central, and 11 central and peripheral. The cavitary defects included 21 peripheral, mostly (17 cases) involving all three walls, 4 central, and 18 peripheral and central deficiencies. The combined defects were usually peripheral and central (28 cases) and peripheral in only 5 cases. Regarding the use of bone transplants, both spongious autografts and allografts were used. The autograft spongiosa was obtained from the resected femoral head in primary implantations and in the form of bone paste from the reaming process in revision surgery, and it was inserted in this form in a total of 8 cases (3 times in primary implantations). However, allograft spongiosa from the bone bank was usually implanted in the form of chips. The following amounts have been used: 32 times 1/4-3/4 of the femoral head (5 × ~/4, 4 × 1/3, 20 × 1/2, 1 × 2/3, 2 × 3/4), 21 times 1 femoral head, 2 times 2 femoral heads. In a total of 39 cases, neither autologous nor homologous spongiosa was used. The clinical follow-up examinations were conducted 3, 6, and 12 m o n t h s after surgery, thereafter annually. In addition to subjective assessment by the patient, clinical evaluation consisted of assessm e n t of pain, ability to walk, and degree of mobility according to the Harris and Merl~ d'Aubign6 hip scores.

The radiological evaluation included radiographs showing anteroposterior and lateral views taken

before surgery and immediately after surgery, at 3 and 6 months, and t h e n at yearly intervals. The postoperative radiograph was used to assess the quality of the primary position of the components. Assessment of the radiological course involved changes of the implant bed like healing of remaining defects, presence or development of radiolucent and sclerotic lines [191, and osteolyses. The allografts were assessed concerning osseointegration, homogenization, sclerosis, resorption, and remodeling. Cup migration was measured according to the m e t h o d of N u n n et al. [20]. All criteria were evaluated in different zones on the cup according to DeLee and Charnley [21]. Because of the oblong shape, the original zone II was divided in two: a (lateral) and b (medial).

Results Clinical results The average preoperative Harris Hip Score for the 102 hips (98 patients) was 51 points (range, 0-90.6 points). The average postoperative score was 82 points (range, 38.7-100 points). The result was excellent (90-100 points) for 27 hips (26.5%), good (80-89 points) for 31 (30.4%), fair (70-79 points) for 26 (25.5%), and poor (<70 points) for 18 (17.6 %). Of the 18 patients w h o had a poor result, 3 experienced complications (1 dislocation, 1 sciatic nerve palsy, 1 major periarticular ossification), which will be discussed in detail later; 2 had a stroke before the revision; 4 were suffering from rheumatoid arthritis; 3 had bilateral congenital hip dysplasia; 2 had major problems with the contralateral hip, 3 experienced a postoperative fracture of the femur; and 1 a fracture of the contralateral femur. After surgery, the Merl~ d'Aubign6 hip score improved from the score achieved before surgery: 18 to 25 points ( m a x i m u m 30 points). In detail from 3.2 to 5.6 for pain relief, from 3.7 to 4.6 for walking ability, and from 4.3 to 4.8 for joint mobility; 78

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patients had no pain at all. Subjectively, 38 results were rated by the patients as very good, 44 as good, 14 as fair, and 6 as poor. Complications included one intraoperative fracture of the ischium, which united within 3 m o n t h s . Two hips had a t e n d e n c y to dislocate, w h i c h completely disappeared 12 weeks after t r e a t m e n t with a H o h m a n n bandage. In two cases partial transient femoral n e r v e palsy and in one transient sciatic nerve palsy occurred. One h e m a t o m a had to be revised. There w e r e two cases with heterotopic ossifications, causing a m o d e r a t e limitation of joint mobility. One cup implant has had to be replaced so far due to infection and two due to early loosening after 7 and 16 m o n t h s (Fig. 2). One of the aseptic loosenings affected a patient with pelvic discontinuity, the other a patient with severe r h e u m a t o i d arthritis w h o had had two revisions. So far, four steins h a v e had to be revised, one in the abovem e n t i o n e d patient with the septic loosening, two in patients with an isolated aseptic loosening of the stem, and one in a patient with r h e u m a t o i d arthritis w h o had had a femoral fracture b e t w e e n the tip of the stem and the femoral c o m p o n e n t of a knee prosthesis.

Radiographical Results The radiological follow-up e x a m i n a t i o n s revealed good osseointegration of the implant in most of the cases (Figs. 3, 4). In 22 cases a remaining defect b e c a m e visible on the postoperative film, usually extending over one or two zones. These defects were completely filled in by b o n e at the last follow-up in 18 cases (Fig. 5). The h o m o l o g o u s allograft used in 55 cases b e c a m e h o m o g e n e o u s in 19 cases, sclerosed in 20 cases, and in 12 cases it was partially r e m o d e l e d and resorbed. In only 4 cases it

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years after operation Fig. 2. Survival until revision for aseptic loosening as the definition of failure and 95 % confidence intervals.



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stayed unchanged. In 25 cups zonal sclerotic lines w e r e apparent, usually not exceeding one zone. In three cases they w e r e complete. Radiolucent lines, which w e r e always c o m b i n e d with sclerotic lines, b e c a m e visible in 18 cases. They w e r e always smaller t h a n 2 m m and only complete in one case (10 × 1 zone of a total of 4, 5 × 2 zones, 2 × 3 zones and 1 x 4 zones). One of these cases was revised because of septic loosening. Radiolucent lines were always seen in patients with m a j o r defects (type III, 7/18; type II, with i n v o l v e m e n t of at least three walls, 7/18; type I, with i n v o l v e m e n t of at least three walls, 4/18), and after the use of large a m o u n t s of allografts (14/18). Only 5 of the radiolucent lines were progressing until the last follow-up. Migration was visible in 10 cases, 9 of which had allografts. All had m a j o r defects (type IV, 2/10; type III, 5/10; type II, with i n v o l v e m e n t of at least three walls, 3/10). Two migrated m o r e t h a n 5 m m and were revised. A n o t h e r two cups s h o w e d a migration b e t w e e n 3 and 5 m m . One of these had a large a m o u n t of allograft (2 heads) and a second revision because of m a j o r protrusion and the other one had a p r i m a r y implantation of the revision cup because of severe protrusion in renal osteopathy. Neither of these two patients was symptomatic. Neither showed other signs of loosening. The r e m a i n i n g 6 cases had m o d e r a t e migration of less t h a n 2 m m w i t h o u t other signs of radiological loosening. They were all asymptomatic. Discussion M a n y different technical approaches have been developed to o v e r c o m e the difficulties in revision arthroplasty of failed acetabular c o m p o n e n t s . The m a i n p r o b l e m is to reestablish and m a i n t a i n stable implant fixation in those cases w h e r e loosening and migration of the device has caused a m a j o r defect and a significant loss of b o n e stock in the acetabulum. Minor defects can usually be supplied with standard implants. In larger defects, additional widening of the acetabular cavity has to be carefully avoided, especially w h e r e the anterior and posterior wall is allready thin or partially missing. This m e a n s that additional b o n e loss can be critical in respect of stable p r i m a r y fixation. An alternative is the use of allografts, but this is controversial. There are strong suggestions that they fail after a certain period of time, especially if t h e y are large and structural [8-13]. Filling the cavities with bone c e m e n t does not allow a biological restoration t h r o u g h n e w b o n e formation. Fragmentation and deterioration of the c e m e n t occurs; serious osteolyses will worsen the situation [71. The results of c e m e n t e d acetabular

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Fig. 3. Preoperative radiograph (A) and postoperative radiographic course (B-D) of a 71-year-old woman supplied with an oblong revision cup after loosening of a cemented cup showing good osseointegration of the implant and remodeling of the allograft.

revisions are not satisfactory [ 17,22-27]. Acetabular meshes also seem to be unsatisfactory [28]. Revisions with supporting rings or shells include the disadvantage of using both cement and allografts and can also show comparatively high failure rates [29]. Results differ for n o n c e m e n t e d revisions using standard acetabular implants [4,30-37], but the devices usually cannot be fixed successfully in large defects without using allografts. The use of a large-diameter standard cup allows better support by the patient's o w n skeletal structures and a lowering of the hip center. But the limiting factor in obtaining contact superiorly with the ilium and inferiorly with the ischium and pubis with a spherical cup is the anteroposterior dimension of the

remaining acetabular bone [38]. Another concept is the use of a bilobed cup w h e r e two spherical components are fused together. Experience with this cup started, to our knowledge, in 1990 and included a comparatively small n u m b e r of patients [39,40]. The implants are comparatively heavy and have thick metal walls that might be theoretically disadvantageous. An additional problem in revision arthroplasty is the reconstruction of the rotation center, which is often superior and lateral displaced, compared with the primary anatomic hip center, due to migration of the loosened cup. This is important because the cup and femoral c o m p o n e n t loosening rate is higher with a superolateral cup position ]41,42]. Standard

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Fig. 4. Pre- (A) and postoperative radiographs (B-D) of a patient with a large oval defect in the presence of loosening of a noncemented polyethylene screw-in cup. Note the osseous integration of the revision cup and the homogenization of the implant bed during the postoperative course.

implants have to be positioned superior unless they rest on bone transplant or cement, which is obviously not ideal. The results obtained so far with the LOR cup show that the objective to o v e r c o m e the abovem e n t i o n e d disadvantages has largely been achieved. The minimal a m o u n t s of transplant bone used in each case, and the large n u m b e r of cases in which no bone grafting was required at all, demonstrate the distinct advantage of avoiding extensive reconstruction of the socket bed with transplanted bone. Supporting the cup on the allograft could also be avoided, which has t u r n e d out to be a recognizable advantage [11,28,43]. Although the ilium thins out superiorly, it was possible to achieve direct contact

and fixation of the implant in this area in all cases, avoiding the use of bulky structural allografts. If the bone stock superiorly does not allow this fixation, a supporting ring with flanges or even a saddle prosthesis are seen as an alternative for reconstruction. These approaches are considered to be the least desirable and were not used in the present series. Nevertheless, the observation that the allografts, w h e n used, were integrated well, and remaining postoperative defects were restored in due course, proves that the implant allows a restoration of the bone stock even after revision surgery, which creates better conditions in case of a re-revision. It is likely that the good ossseointegration of the implants which was to be seen in the majority of

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Fig. 5. Radiographic course (A-D) of an implant over 5 years showing remaining defect at the acetabular roof after surgery (B) completely restored at the last follow-up (D).

cases, has been p r o m o t e d by use of p u r e coarseblasted titanium, a material believed to h a v e good properties with regard to biological reaction ]44,45] and osseointegration [46,47]. A course blasted surface appears to be best for initial b o n e ongrowth ]48,49]. The initial anchorage of the implant was optimized by screw fixation, and p r i m a r y stability was achieved in all cases, enabling early mobilization of the patients. This allowed osseous bridging of defects, which occurred after the short time of 6 to 12 m o n t h s after surgery. The a d v a n t a g e of using screws for p r i m a r y stable fixation in revision arthroplasty has also b e e n reported f r o m other authors ]30,33,35]. The possible disadvantage of fretting can occur in any cup secured with screws,

and might cause significant w e a r debris, leading to local osteolyses [501. This p h e n o m e n o n has not been observed thus far in the present study. Its occurrence cannot be excluded, but in our opinion it has less significance than an insufficient p r i m a r y stability, which might be caused by dispensing screws. Injury to blood vessels while placing the screws is a general p r o b l e m using screw fixation for an acetabular c o m p o n e n t [51,52[. Because of the special anatomy, this risk is probably higher in revision surgery [53]. By careful p l a c e m e n t of the screws w i t h o u t medial extraosseous penetration, injury to blood vessels could be completely avoided so far. The clinical results with the LOR cup c o m p a r e v e r y favorably w i t h o t h e r concepts that were evaluated after a similar follow-up period [11,30,

Oblong Cups for LargeAcetabular Defects • 33,35,54]. The radiological results revealed comparatively few cases with radiographic signs that might indicate suboptimal osseointegration. The presence of radiolucent lines can be c o u n t e d as a sign of loosening if they are complete, progressive, and extending 2 mm. In the present study they were never wider t h a n 2 mm, in only one case complete and in only five cases partially progressing. However, even the total n u m b e r of radiolucent lines (17.6%) is low in comparison with other series [12,16,17,23,24,30,32-35,37,54,55]. The fact that they were visible mainly in patients w h o were grafted a n d / o r had a major defect indicates that bone ingrowth appears m o r e unlikely in these cases. But a zonal radiolucency is still compatible with a well-fixed c o m p o n e n t as h u m a n retrieval data have s h o w n [56,57]. Migration could be seen in 9.8% of our investigated cases. But it only indicates loosening if it is progressive and exceeding 5 m m [58,59]. The two cases with major migration (<5 mm) were evaluated as radiologically loose and were revised, although clinical symptoms were absent. Both were exceptional cases, as mentioned above. Conceming the two cases with moderate migration (3-5 mm) it has to be taken into account that one had a re-revision and a major amount of allograft and the other a renal osteopathy, a bone disease that promotes protrusion and migration. The remaining cases also had a higher risk for migration because they were all allografted and had large defects. Nevertheless the incidence of migration in the present study is comparatively low [4,11,12,17,23,28, 32, 58,60]. Also low is the total number of aseptic loosenings, which can be defined as definitive failure [4,11,12,17, 23,25-27,29,32,36,43,57,61,62]. In conclusion, the demands placed on a m o d e r n revision implant appear to have been met by the LOR cup and the results achieved to date justify further clinical application.

References I. Willert HG, Ludwig J, Semlitsch M: Reactions of bone to methacrylate after hip arthroplasty: a long term gross, light microscopic, and scanning electron microscopic study. J Bone Joint Surg [Aml 56:1368, 1974 2. Willert HG, Buchhom GH, Hess T: Die Bedeutung von Abrieb und Materialerm6dung bei der Prothesenlockerung an der Htifte. Orthop~de 18:350, 1989 3. Morscher E, Dick W, Seelig W: Revision arthroplasty of the hip joint with autologous and homologous cancellous bone. Orthopfide 18:428, 1989 4. Engh CA, Glassmann AH, Griffin WL, Mayer JG: Cementless revision for failed cemented total hip arthroplasty. Clin Orthop 235:91, 1988

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