The use of cement for fixation of the acetabular component in total hip arthroplasty is controversial. Three generations of cement technology have not been able to reduce the problem of loosening of the acetabular component despite tremendous improvements in femoral loosening rates. The three generations of cement technique 8 have most notably been described with respect to the femoral side of total hip arthroplasty. First generation techniques include finger packing and bowl mixing, no intramedullary plug, no pressurization of the cement, and limited sizes and geometry of components. Second generation techniques involve the use of an intramedullary plug in the femoral canal, cleaning of the bone bed, drying of the bone, retrograde cement insertion, and the availability of multiple sizes and geometries of femoral and acetabular components to better match patient geometry. The third generation technique includes second generation techniques with the addition of vacuum mixing and porosity reduction in the cement, pressurization of the cement mantle after insertion, and centralization of the femoral stem within the cement mantle.
Several studies have addressed the relationship between cementing techniques and radiolucencies in cemented acetabular components and have shown that there is a prognostic significance to Zone 1 radiolucencies in the long-term survival of cemented acetabular cup. 3,9,11,12 However, to the authors knowledge, no study has evaluated the effect of porosity reduction techniques in the cement on the incidence of radiolucencies in the cemented acetabulum.
The purpose of the current study was to determine whether porosity reduction techniques in preparing the cement reduced the incidence of radiolucent lines in Zone 1 in cemented acetabular cups on initial radiographs obtained postoperatively.
MATERIALS AND METHODS
Between 1970 and 1998, 2237 consecutive cemented acetabular cups were performed by one surgeon (MAR). Initial radiographs were reviewed and evaluated for radiolucencies in Zone 1 by the senior author and surgeon (MAR). No interobserver or intraobserver study of variability was done. Loosening was evaluated radiographically in all patients. Radiographically loose cups were defined as having 100% radiolucency or migration of the cup. Components that were being revised or were radiographically loose were considered failures.
The 2237 cemented cups were separated into five groups based on the cementing techniques used. Group 1 had bowl mixing of the cement, preservation of subcondral bone, use of three or four half-inch drill holes, and finger packing of cement. Group 2 had gouging of the acetabulum with an osteotome or curette, water pick lavage, bowl mixing of the cement, preservation of subcondral bone, use of three or four half-inch drill holes, and finger packing of cement. Group 3 had preservation of subcondral bone, use of three or four half-inch drill holes, gouging of the acetabulum with an osteotome or curette, water pick lavage, and local pressurization with a cement gun. Group 4 had preservation of subcondral bone, gouging of the acetabulum with an osteotome or curette, use of multiple 5-mm drill holes, water pick lavage, drying the acetabulum, hypotensive anesthesia, porosity reduction of either centrifuge or vacuum mixing of the cement, and finger packing of cement with local pressurization with a cement gun. Group 5 had bowl mixing, water pick lavage, drying the acetabular bone, hypotensive anesthesia, multiple 5-mm drill holes, total pressurization of cement into socket with rubber dam, removal of sclerotic cortical bone in Zone 1, reaming into cancellous bone, and total containment of the cup within the bony socket (Table 1). Group 4 is the only group in which porosity reduction techniques were used in the preparation of the cement. The percentage of cups with Zone 1 radiolucencies was observed in the five groups.
Various types of prostheses were used throughout the 28-year period. Between 1970 and 1974, 185 cemented Charnley (Thackery, London, England) total hip replacements with ram extruded all polyethylene acetabular components were performed. Between 1974 and 1980, 440 T-28/TR-28 (Zimmer, Warsaw, IN) total hip replacements with compression molded all polyethylene acetabular components were done. There were 238 MOSC (Miami Orthopedic Specialist Clinic, Biomet, Warsaw, IN) total hip replacements performed between 1980 and 1983. One hundred thirty-eight of these had metal-backed compression molded polyethylene acetabular components, and 100 had compression molded all polyethylene acetabular components. Between 1984 and 1989, 512 metal-backed polyethylene cups were placed with MOSC femoral stems. From 1989 to the present time, all polyethylene direct compression molded cups (Biomet) have been used for cemented sockets. The percentage of failures was calculated for each type of prosthesis; however, the periods for different cement techniques do not correspond to the period for each type of prosthesis.
Table 1 shows that the lowest percentage of Zone 1 radiolucencies was in Group 5, in which porosity reduction techniques were not used. There were 245 cups in Group 1. Forty of the 245 cups (22%) had radiolucencies in Zone 1 on initial radiographs obtained postoperatively. There were 380 cups in Group 2. One hundred fourteen of the 380 cups (30%) had Zone 1 radiolucencies on radiographs obtained postoperatively. There were 238 cups in Group 3. Seventy-six of these 238 cups (32%) had Zone 1 radiolucencies. There were 916 cups in Group 4. Two hundred two (22%) of these cups had Zone 1 radiolucencies on initial radiographs obtained postoperatively. There were 458 cups in Group 5. Of these 458 cups, 52 (11%) had radiolucencies in Zone 1.
Table 2 shows that the lowest failure rate of cemented acetabular cups with adequate long-term followup was in the patients who received Charnley prostheses. This group did not have porosity reduction as part of the cementing technique. Of 185 Charnley cups inserted, three were loose and nine were revised for a failure rate of 6.4%. Of the 440 T-28/TR-28 cups, 31 were loose and 35 were revised for a failure rate of 15%. Of the 100 MOSC all-polyethylene cups, 11 were loose and four were revised for a failure rate of 15%. Of the 138 MOSC metal-backed polyethylene cups, 11 were loose and 20 were revised for a failure rate of 22%. Of the 512 metal-backed polyethylene cups, 64 were loose and 110 were revised for a failure rate of 34%. None of the 404 all-polyethylene components placed between 1989 and 1994 were loose and six were revised for a failure rate of 1%. One of the 458 all-polyethylene cups placed between 1995 and 1998 was loose and none were revised.
The purpose of the current study was to determine whether the addition of third generation, porosity reduction techniques, helped reduce the incidence of radiolucent lines in Zone 1 of cemented acetabular components in total hip arthroplasty. The results of this study indicate that the addition of third generation cement techniques did not significantly reduce the incidence of Zone 1 radiolucencies in cemented acetabular cups. The group with the lowest percentage of Zone 1 radiolucencies was Group 5 in which bowl mixing of the cement was used. The Charnley prostheses in which simple, first generation techniques were used had the lowest failure rate.
The authors acknowledge the limitations of a retrospective study. In addition, the lack of interobserver and intraobserver variability studies on the radiographic interpretations is a concern. However, despite these limitations, the results are consistent and of value.
Russotti et al 14 initially reported that improved cementing techniques had reduced femoral and acetabular loosening rates in a short-term followup series. However, Mulroy and Harris 7 reported that at 11 years the femoral loosening rate improved with newer cement techniques whereas the acetabular loosening rate still was 42%. In a 15-year followup study from the same institution, the acetabular loosening rate was 52%. 6 For both of these studies, the acetabular cement technique involved preservation of the subchondral bone, three or four 12.5-mm anchor holes, and finger packing without pressurization.
Bourne et al 2 evaluated the results of cemented total hip replacements in which second generation cement techniques were used and reported that only 9% of the acetabular components were radiographically loose at 10 to 15 years. This was significantly different from the loosening rates reported by Mulroy and Harris. 7 The authors thought that their improved rate of acetabular loosening was attributable to the use of cement techniques that included preservation of the subchondral bone, multiple small drill holes, debridement with lavage irrigation, drying with thrombin, and pressurization of the cement.
Ranawat et al 11 analyzed radiolucent lines around acetabular components that were cemented using “modern methods of cement technique.” They reported that the cumulative amount of radiolucencies were reduced significantly using the newer techniques of pulsed lavage irrigation, preservation of the subchondral bone, pressurization of the cement, use of cement in a low viscosity state, and multiple drill holes into the cancellous bone. They also used hypotensive anesthesia to reduce the amount of bleeding at the bone-cement interface and emphasized that less than 1 cm of the superior aspect of the cup should be left uncovered. More recently, Ranawat et al 9 reported 9-year followup on a series of 236 total hip replacements with cemented acetabular components using these same cementing techniques. They reported a 2.2% rate of acetabular loosening if radiolucent lines were not observed on the radiographs obtained immediately after surgery versus a 14.4% rate of loosening if radiolucent lines were present in Zone 1 on the first radiographs obtained postoperatively. Garcia-Cimbrelo et al 3 also found that cemented acetabular components with radiolucency on the initial radiograph obtained postoperatively had a higher rate of loosening. It seems that the presence of radiolucent lines on initial radiographs obtained postoperatively is related directly to increased rates of loosening of the acetabular component. This also has been well recognized on the femoral side. The authors consider radiolucent lines to be a technique-related phenomenon of bone preparation, patient selection, cement pressurization, and control of bleeding at the bonecement interface during cement and component insertion.
The results of Ranawat et al 9 and Garcia-Cimbrelo et al 3 regarding the prognostic significance of radiolucency about the acetabulum on the first radiographs obtained post-operatively have been confirmed by a recent investigation at the authors’ institution. 12 In this study the most important factor for long-term survival of the acetabular component was the presence of radiolucency in Zone 1 on the initial radiograph obtained after surgery. Based on these studies, it seems that cementing technique is vitally important to the long-term survival of a cemented acetabular component. Furthermore, the above series show that the success of the acetabular component can be predicted from the first radiographs obtained postoperatively instead of waiting for 15 to 20 years followup.
Because of the past loosening rates in cemented acetabular components, the technology has improved in the use of cementless sockets. Unfortunately, the wear rates of modular cementless cups have been reported to be three to fives times higher than those of cemented all-polyethylene cups. 5,10 This increased wear contributes to increased osteolysis seen in uncemented cups. In addition, backside wear of the polyethylene in modular acetabular components is a significant contributor to osteolysis and subsequent loosening. 4,13
The current authors think that cemented, direct compression molded acetabular components offer an advantage of reduced wear and reduced osteolysis, which provide good longevity. However, cementing an acetabular component is very technique sensitive and patient dependent. Credit should be given to Ranawat et al 11 and Bourne et al 2 for developing second generation cementing techniques for the acetabulum including multiple perforations of the subchondral bone, drying the bone bed, and pressurization of the cement into the entire acetabulum at the same time. They did not use centrifugation or vacuum mixing and had good results with cementing the acetabulum. The results of the current study confirm that porosity reduction is not the essential factor in cementing an acetabular cup. The authors think that the high failure rates reported previously 6,7 are attributable to the cementing techniques used. Second generation cementing techniques were not used in that the bone bed was not dried, the subchondral bone was not perforated, and the cement was not pressurized.
If good impregnation can be obtained, then radiolucent lines are eliminated. The lack of radiolucent lines is an indication that good cementing technique was used and the long-term survival of the socket is increased. The ability to achieve a good cement mantle in the acetabulum is dependent on several factors. The surgeon should first have a good cancellous bone bed in the acetabulum. Patients with conditions such as avascular necrosis, 15 medial protrusio osteoarthritis, 1 and patients who are young have poor results with cemented acetabular components because lack of cancellous bone. To obtain a good cancellous bed, the acetabulum must be reamed down through the subchondral bone.
Once a good cancellous bone bed is achieved by reaming, the success of the cementing technique is dependent on the preparation of the acetabulum by hypotensive anesthesia, pulsed lavage, drying with thrombin or vasoconstrictors, perforating localized sclerotic regions, cleaning out all soft tissue from cysts, and good uniform pressurization of the cement in the entire socket at the same time. This last factor is very important because localized pressurization such as that performed with the tip of a cement gun just results in blood being extruded out of the surrounding area of bone ruining the cleaning effect of the water pick lavage and resulting in loss of the ability to get good cement impregnation.
Reaming down through the subchondral bone into the cancellous bone also results in a deeper socket, which allows the acetabular component to be seated completely within the confines of the bony acetabulum. Ranawat et al 11 alluded to this necessity when they stated that the acetabular component must have less than 1 cm uncovered by bone. Sarmiento et al 16 confirmed the importance of complete containment of the component within the bony acetabulum. This was a benefit that was not appreciated in the early Charnley acetabular com-ponents because the components only had an outside diameter of less than 44 mm. The authors think that the success of early Charnley cemented cups in the current series using simple cement techniques can be attributed to the fact that there were only two sizes available, which led to complete containment within the acetabulum. The importance of full cup containment also can be seen in cases of developmental dysplasia of the hip. Cemented acetabular cups do poorly in dysplastic hips that have peripheral bone defects of the acetabulum.
Good, meticulous second generation cement techniques are absolutely essential for the acetabulum because they allow for the needed cement microinterlock that good impregnation into the cancellous bone provides. There are five factors that are essential for the long-term success of cemented acetabular components: (1) Good cancellous acetabular bed with particular attention given to removing sclerotic areas from Zone 1; (2) Complete coverage of the component down within the bony acetabulum; (3) Jet pulsed lavage irrigation of cancellous bone; (4) Drying of the acetabulum with a sponge soaked with thrombin or a vasoconstricting agent; and (5) Pressurization of the entire acetabulum at the same time.
The results of the current study show that the lowest failure rate was in Charnley cemented acetabular components using first generation cement techniques. Again, the authors theorize that this is because only two sizes were available that allowed for complete cup containment. The results of the current study showed that the addition of porosity reduction techniques did not lower failures or Zone 1 radiolucencies. Only after the technique of reaming down into the cancellous bone and meticulously removing sclerosis in Zone 1, cleaning the acetabulum, obtaining complete coverage, and using complete pressurization did the incidence of Zone 1 radiolucencies decrease on the initial radiographs obtained postoperatively. Furthermore, these results were obtained after discontinuation of the use of centrifuge and vacuum mixing. The authors conclude that third generation cement techniques are not necessary for cemented acetabular components. It is the biology and the preparation of the bone that are important, not the preparation of the cement.
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Richard F. Santore, MD; and Richard A. Brand, MD, Guest Editors