Revision total hip arthroplasty with retained acetabular component

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1 Washington University School of Meicine Digital Open Access Publications 2014 Revision total hip arthroplasty with retaine acetabular component Muyibat A. Aelani Washington University School of Meicine in St. Louis Nathan A. Mall McBrie & Sons Humaa Nyazee Washington University School of Meicine in St. Louis ohn C. Clohisy Washington University School of Meicine in St. Louis Robert L. Barrack Washington University School of Meicine in St. Louis See next page for aitional authors Follow this an aitional works at: Recommene Citation Aelani, Muyibat A.; Mall, Nathan A.; Nyazee, Humaa; Clohisy, ohn C.; Barrack, Robert L.; an Nunley, Ryan M.,,"Revision total hip arthroplasty with retaine acetabular component." ournal of Bone an oint Surgery.96, (2014). This Open Access Publication is brought to you for free an open access by Digital It has been accepte for inclusion in Open Access Publications by an authorize aministrator of Digital For more information, please contact

2 Authors Muyibat A. Aelani, Nathan A. Mall, Humaa Nyazee, ohn C. Clohisy, Robert L. Barrack, an Ryan M. Nunley This open access publication is available at Digital

3 1015 COPYRIGHT Ó 2014 BY THE OURNAL OF BONE AND OINT SURGERY, INCORPORATED Revision Total Hip Arthroplasty with Retaine Acetabular Component Muyibat A. Aelani, MD, Nathan A. Mall, MD, Humaa Nyazee, MPH, ohn C. Clohisy, MD, Robert L. Barrack, MD, an Ryan M. Nunley, MD Investigation performe at the Department of Orthopaeic Surgery, Washington University School of Meicine, St. Louis, Missouri Backgroun: Aseptic an osteolysis commonly limit the survivorship of total hip prostheses. Retention of a well-fixe acetabular component, rather than full acetabular revision, has multiple avantages, but questions have lingere regaring the clinical success an prosthetic survivorship following this proceure. We examine the impact of acetabular component position, polyethylene type, liner insertion technique, femoral hea size, an simultaneous revision of the entire femoral component (as oppose to hea an liner exchange) or bone-grafting on mi-term to long-term prosthetic survival following such limite revisions. Methos: One hunre hips in 100 patients with osteolysis, polyethylene wear, or femoral component unerwent revision total hip arthroplasty with retention of the acetabular component. Acetabular component inclination an anteversion were measure on prerevision raiographs an were categorize accoring to preetermine positional safe zones (inclination of 35 to 55 an anteversion of 5 to 25 ). Operative reports were reviewe for femoral hea size, polyethylene liner type (conventional or highly cross-linke), liner insertion technique (use of the existing locking mechanism or cementation), whether the patient ha revision of the entire femoral component, an use of bone graft. Outcomes of interest inclue the Harris hip score, University of California at Los Angeles (UCLA) activity score, episoes of instability, an nee for repeat revision. Results: At an average of 6.6 years (range, two to fourteen years) postoperatively, the Harris hip an UCLA activity scores were both significantly improve compare with the preoperative scores (p < an p < 0.01, respectively). Overall, the failure rate was 13%. In aition, 6% of the patients ha postoperative instability. Hips in which the acetabular component was outsie of the safe zone for inclination ha a higher rate of failure (p = 0.048). Use of conventional, rather than highly cross-linke, polyethylene at the time of revision was also associate with an increase rate of repeat revision (p = 0.025). Conclusions: Revision total hip arthroplasty with retention of the acetabular component is associate with goo outcomes in hips with an appropriately positione, well-fixe acetabular component. Acetabular components outsie the safe zone for inclination were at a higher risk for failure, as was use of conventional polyethylene. Level of Evience: Therapeutic Level IV. See Instructions for Authors for a complete escription of levels of evience. Peer Review: This article was reviewe by the Eitor-in-Chief an one Deputy Eitor, an it unerwent bline review by two or more outsie experts. The Deputy Eitor reviewe each revision of the article, an it unerwent a final review by the Eitor-in-Chief prior to publication. Final corrections an clarifications occurreuring one or more exchanges between the author(s) an copyeitors. Although total hip arthroplasty is one of the most successful surgical interventions, the results can be compromise by infection, component malposition, an aseptic. The major cause of aseptic failure is, often seconary to osteolysis 1. In patients with osteolysis etecte prior to acetabular component, retention of the acetabular component has multiple avantages, incluing ecrease morbiity an preservation of pelvic bone stock 2-4. Structurally stable lytic efects can be aresse with bonegrafting through screw holes or aroun the periphery of a wellfixe acetabular component 5. Aggressive ebriement an curettage of these lesions in combination with exchange of the Disclosure: None of the authors receive payments or services, either irectly or inirectly (i.e., via his or her institution), from a thir party in support of any aspect of this work. One or more of the authors, or his or her institution, has ha a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomeical arena that coul be perceive to influence or have the potential to influence what is written in this work. No author has ha any other relationships, or has engage in any other activities, that coul be perceive to influence or have the potential to influence what is written inthis work. The complete Disclosures of Potential Conflicts of Interest submitte by authors are always provie with the online version of the article. Bone oint Surg Am. 2014;96:

4 1016 TABLE I Postrevision Instability Failure Acetabular Inclination (eg) Acetabular Anteversion (eg) Femoral Hea Size (mm) Stem Revision Rerevision Reason Time After Revision (mo) No No Not applic. Not applic Yes No Not applic. Not applic No No Not applic. Not applic No Yes Instability Yes Yes Instability No Yes Instability 13 polyethylene liner to a highly cross-linke polyethylene liner may be sufficient to halt the lytic process 5-7. Proponents of this approach reserve full acetabular revision for patients with a loose or malpositione acetabular component. Others, however, recommen full acetabular revision espite aequate component position an stability, citing higher rates of islocation an rerevision with retention of the acetabular component The purpose of the current stuy was to etermine the mi-term to long-term outcomes an prosthetic survivorship after revision total hip arthroplasty with retention of a well-fixe acetabular component in patients with pelvic osteolysis an/or polyethylene wear. We aime to etermine whether acetabular component position, polyethylene type, liner insertion technique, femoral hea size, an simultaneous revision of the entire femoral component (as oppose to hea an liner exchange) or bone-grafting are potential risk factors for failure. Materials an Methos Following approval by our institutional review boar, our institution s joint replacement registry was reviewe for all cases of revision total hip arthroplasty performe from 1996 to Only those with a preoperative iagnosis of pelvic osteolysis, polyethylene wear, or aseptic associate with osteolysis an/or substantial wear were inclue. Cases in which the acetabular component ha been revise or acetabular component ha been ocumente were exclue. Patients were followe for a minimum of two years, or until rerevision or TABLE II Review of Failures Acetabular Inclination (eg) Acetabular Anteversion (eg) Polyethylene Type Femoral Hea Size (mm) Stem Revision Cemente Liner Bone- Grafting Reason Failure Time After Revision (mo) Conventional 26 No No Yes Instability Highly 36 Yes Yes No Instability 3 cross-linke Conventional 32 No No Yes Instability Conventional 26 No No Yes Acetabular Conventional 28 No No Yes Acetabular Highly 28 No Yes No Fracture liner 22 cross-linke Conventional 28 No No Yes Osteolysis Conventional 28 Yes No Yes Infection Conventional 36 Yes No No Femoral Conventional 28 No No Yes Acetabular Conventional 36 No No Yes Osteolysis Conventional 28 No No Yes Acetabular Conventional 32 No No Yes Osteolysis 168

5 1017 TABLE III Patient Outcomes Points Preoperative Postoperative P Value Harris hip score Overall score < Pain subscore < UCLA activity score <0.01 eath. Surviving patients who ha not unergone rerevision an ha not been evaluate within the six months preceing the stuy were contacte by telephone to upate the Harris hip an University of California at Los Angeles (UCLA) activity scores an to inquire about interim episoes of instability or reoperations. Of the 143 hips that were revise because of pelvic osteolysis, polyethylene wear, or femoral uring the perio of stuy, twenty-three (16%) in twenty-three patients were lost to follow-up less than two years postoperatively an four (3%) in four patients were followe for two or more years but coul not be reache by telephone for a clinical upate. Fifteen patients (sixteen hips; 11%) ie prior to the two-year follow-up. Thus, 100 hips (70% of the 143) in 100 patients remaine for analysis. Of these 100 hips, eighty-seven in eighty-seven patients ha at least two years of clinical follow-up; seventy-nine of these patients were examine in person less than six months before the stuy an eight were reache by telephone for a clinical upate. The remaining thirteen hips in thirteen patients ha unergone rerevision. Operative reports were reviewe for etails of the proceure, incluing surgical approach, femoral hea size, polyethylene liner type (conventional or highly cross-linke polyethylene), liner insertion technique (use of the existing locking mechanism or cementation), whether the patient ha a simultaneous femoral revision, an use of bone graft aroun the acetabular an/or femoral component. Clinical charts were examine for emographic information an the prevalence of postoperative complications, specifically islocations an the nee for aitional revision. Clinical outcomes were measure with the Harris hip score an UCLA activity score obtaine at the time of the most recent follow-up. Repeat revisions, performe for any reason, were efine as failures in this stuy. This cohort inclue fifty-four women an forty-six men. The average age at the time of revision was sixty years (range, twenty-two to eighty-nine years). The inex revisions were preominantly performe for osteolysis (59%: acetabular osteolysis only in 39%, femoral only in 11%, an both in 9%) an aseptic of the femoral component (33%). There were sixty-two isolate hea an liner exchanges an thirty-eight femoral component revisions for ue to osteolysis. All femoral component revisions also inclue exchange of the femoral hea an polyethylene liner. All proceures were performe through a posterior surgical approach. All acetabular components were well fixe at the time of surgery. Preoperative anteroposterior pelvic raiographs as well as preoperative anteroposterior an cross-table lateral raiographs of the affecte hip were reviewe for all patients for whom they were available. If they were unavailable, raiographs from the first post-proceure follow-up visit were measure. Since all acetabular components were well fixe an retaine, we assume that there was no important change in acetabular component position between the preoperative an immeiate postoperative perios. Inclination an anteversion of the acetabular component were measure on the anteroposterior pelvic an cross-table lateral raiographs, respectively, as previously escribe 12,13. On the anteroposterior pelvic raiograph, a line was rawn across the ischial tuberosities to estimate the orientation of the pelvis. The angle between this line an a line along the opening of the acetabular component represents inclination of the acetabular component 13. The raiographic safe zone for inclination for this stuy was efine as 35 to On the cross-table lateral raiograph, acetabular component anteversion was measure as the angle between the long axis of the acetabular opening an a line perpenicular to the long axis of the boy 12. The anteversion safe zone for this stuy was efine as 5 to All raiographs were measure by the same bline reviewer (M.A.A.). A secon bline observer (N.A.M.) reviewe the raiographs of a ranom subset of twenty-five patients (25%) in this cohort, an interrater reliability was etermine by calculating the intraclass correlation coefficient (ICC) with use of a two-way mixe-effects moel. The agreement between the reviewers was excellent, with an ICC of 0.93 for the anteversion assessment (95% confience interval [CI], 0.86 to 0.96; p < 0.001) an an ICC of 0.90 (95% CI, 0.82 to 0.95; p < 0.001) for the inclination assessment. Patient outcomes were etermine by comparing the prerevision an postrevision Harris hip an UCLA activity scores. Mean outcome scores were calculate, an a two-taile t test was utilize to assess ifferences between prerevision an postrevision scores. We examine several risk factors for failure requiring aitional surgery: (1) acetabular component inclination an anteversion, (2) femoral hea size, (3) polyethylene liner type (conventional or highly cross-linke polyethylene), (4) liner insertion technique (use of the existing locking mechanism or cementation of a new liner), (5) whether the patient ha revision of the entire femoral component, an (6) use of bone graft for any pelvic or femoral osteolytic efects. These variables were recore for each patient an analyze for association with failure in a multiple logistic regression analysis. All p values of <0.05 were consiere significant. Statistical analyses were performe with use of SPSS for Winows, version 20 (IBM, Armonk, New York). Source of Funing There were no external sources of funing for this stuy. Results Six patients (6%) ha postoperative instability; one sense subluxation without any true islocation, an five ha one or more islocations requiring close reuction (Table I). Four of the six ha acetabular anteversion that was outsie of the safe TABLE IV Risk Factors for Failure Risk Factor Os Ratio (95% CI) P Value Acetabular component 4.6 ( ) inclination Acetabular component ( ) anteversion Conventional polyethylene 14.8 ( ) Liner insertion technique 1.52 ( ) 0.75 Femoral hea size 3.52 ( ) 0.17 Complete femoral 3.32 ( ) 0.16 component revision Bone-grafting 0.75 ( ) 0.74

6 1018 Fig. 1 Thirty-nine percent of the acetabular components fell within the raiographic safe zone for anteversion an inclination (shae area). range. One of the six ha greater than acceptable acetabular inclination. One of the six ha a femoral hea that was <28 mm. Thirteen hips (13%) require another revision: five because of (acetabular in four an femoral in one) at an average of sixty-six months after revision, three because of recurrent instability at an average of 5.6 months postoperatively, three because of progressive osteolysis aroun a fixe acetabular component at an average of 123 months, one because of a fracture liner at twenty-two months, an one because of infection at forty months (Table II). Of the twelve patients with repeat revision for noninfectious causes, eight unerwent full acetabular component revision, two unerwent full femoral component revision, an two ha a repeat hea-liner exchange. Eighty-seven hips (87%) in the cohort ha not require rerevision at the time of follow-up, at an average of 6.6 years (range, two to fourteen years). The Harris hip scores before revision average 60.1 points (range, 28 to 100 points), with significant improvement to an average of 77.7 points (range, 20 to 100 points) postoperatively (p < ). The pain subscore of the Harris hip score also improve significantly, from 22.0 to 36.6 points (p < ). The average UCLA score before revision was 4.4 points (range, 2 to 10 points), which improve to an average of 5.3 points (range, 2 to 10 points) postoperatively (p < 0.01) (Table III). The mean acetabular component inclination for all hips was 44.2 (range, 26 to 65.5 ; stanareviation [SD], 8.9 ) an the mean anteversion was 22.6 (range, 210 to 71 ; SD, 12.4 ). Seventy-three hips (73%) were insie the inclination safe zone (35 to 55 ). Fifty-six hips (56%) were insie the anteversion safe zone (5 to 25 ). Thirty-nine hips (39%) were insie the safe zone for both parameters (Fig. 1). Acetabular components outsie of the inclination safe zone were 4.6 times more likely to fail than were those within the safe zone (95% CI, 1.01 to 20.8; p = 0.048). However, there was no significant association between acetabular anteversion an failure (p = 0.053) (Table IV). A highly cross-linke polyethylene liner was use in 56% of the revisions in this stuy. Eleven of the thirteen hips requiring repeat revision ha been initially revise with a conventional polyethylene liner. The risk of failure in the patients who receive a conventional polyethylene liner at the time of the initial revision was 14.8 times higher than that in the patients who receive a highly cross-linke polyethylene liner (95% CI, 1.4 to 155.4; p = 0.025). The reasons for failure following the use of conventional polyethylene were acetabular in four hips, osteolysis aroun a fixe acetabular component in three, instability in two, femoral in one, an infection in one. The existing locking mechanism was intact an utilize for liner insertion in sixty-two cases. In the remaining thirty-eight cases, the locking mechanism was amage or ha a poor track recor an therefore a new liner was cemente into the existing acetabular component. The metho of liner insertion was not associate with repeat revision (p = 0.75). The femoral hea utilize at the time of revision was 32 mm in seventy-eight hips an >32 mm in twenty-two hips. Femoral hea size was not associate with failure (p = 0.17). Bone-grafting of periprosthetic lytic lesions was performe in sixty-eight cases; forty hips ha acetabular grafting alone, sixteen ha femoral grafting alone, an twelve ha grafting at both sites. Bone-grafting was not associate with failure (p = 0.74). Thirty-eight hips unerwent revision of the entire femoral component with a polyethylene liner exchange. The average postoperative Harris hip score for those with a femoral revision was 72.3 points compare with 81.3 points for those with a hea-liner exchange alone (p = 0.03). The two groups ha similar Harris hip pain subscores (34.4 an 38.3 points, p = 0.07) an similar UCLA activity scores (4.9 an 5.6 points, p = 0.15). Femoral component revision was not associate with instability or failure (p = 0.80 an p = 0.16, respectively). Discussion This stuy emonstrate a 13% failure rate following revision total hip arthroplasty with retention of a well-fixe acetabular component, with a 3% rate of rerevision for instability. In a previous stuy of 318 hips treate with isolate hea an liner exchange, the rate of repeat revision was 16%, with a 5% rate of repeat revision for instability; however, component position was not reporte 9. Another recent stuy without ata on component position ientifie a 13% rate of repeat revision, with a 5% prevalence of instability requiring revision, in a group of 187 isolate polyethylene exchange proceures followe for an average of 4.2 years 16. Other previous stuies have emonstrate substantially higher rates of instability (range, 15.5% to 29% 4,8,10 ) an failure (25% 10 ) following polyethylene liner exchange. This variability in outcomes may be ue to ifferences in patient populations, surgical techniques, an postoperative rehabilitation. Some stuies, incluing ours, limite their cohorts to patient who unerwent revision because of osteolysis or polyethylene wear 4,8,16, while others inclue patients who unerwent revision for other reasons, incluing instability 10. Also, ifferences in polyethylene may explain higher failure rates, as the use of highly cross-linke polyethylene was not specifically mentione in the previous stuies 4,8-10. Despite the variability in previous stuies, the failure rate in our cohort is comparable with previously reporte failure

7 1019 rates following full acetabular revision. Lie et al. reporte an 11.8% rerevision rate following complete revision of a fixe acetabular component 9 ; Talmo et al. reporte a 15% rate 10.Other authors have emonstrate similar failure rates following liner exchange an complete acetabular revision 17,18. Restrepo et al. compare thirty-six patients who ha unergone polyethylene exchange with thirty-one patients who ha ha revision of a fixe acetabular component an foun similar rerevision rates (8% compare with 3%, p = 0.62) 17.Kohetal.alsofoun comparable rerevision rates between these two cohorts (2.9% compare with 2.2%) 18. Asie from instability an failure, complications are rarely reporte following revision with or without exchange of the acetabular component. Our stuy ha an overall complication rate of 9%, which inclue five islocations, two hematomas, one intraoperative fracture, an one infection. Koh et al. reporte similar complication rates following hea-liner exchange an complete acetabular revision (26.7% compare with 22.8%) 18. They iemonstrate, however, that bloo loss following hea-liner exchange was significantly lower than that after acetabular revision (680.0 ml compare with ml, p = 0.017) an the length of hospital stay was significantly shorter (9.6 ays compare with 12.1 ays, p < 0.001). Some authors have state that the best caniates for acetabular component retention with liner exchange are patients with a well-fixe, well-positione acetabular component 2,3,17,18 ; however, there is not a large amount of ata supporting this. Two stuies showe no ifference in acetabular inclination or anteversion between hips that islocate following hea-liner exchange an those that remaine stable, but the authors i not evaluate the impact of acetabular component position on prosthetic survivorship 8,10. To our knowlege, our stuy is the first to emonstrate an effect of acetabular component position on prosthetic survivorship in the revision setting. Our finings affirm that component position, which is known to limit the survivorship of primary total hip prostheses, also limits the survivorship of revision total hip prostheses with a retaine acetabular component. The current stuy emonstrate an increase risk of failure with use of conventional polyethylene. Highly crosslinke polyethylene was not commercially available or reaily use at our institution until September Prior to that, all revisions were performe with conventional polyethylene. Following that ate, conventional polyethylene liners were use only if the locking mechanism on the retaine acetabular component was salvageable an the manufacturer i not make a highly cross-linke polyethylene option for implantation. Oler implants may not allow for implantation of highly cross-linke polyethylene with the existing locking mechanism. Our results support cementing a highly cross-linke polyethylene liner into those acetabular components rather than implanting a corresponing conventional polyethylene liner into a well-positione, well-fixe acetabular component. Cementation of a liner was not associate with an increase risk of failure, a fining consistent with those of other stuies Given the emonstrate benefits of highly cross-linke polyethylene, incluing the potential to halt osteolysis 23,itis preferable to cement a highly cross-linke polyethylene liner if there is no commercially available highly cross-linke option for the existing, well-fixe acetabular component. Both the Harris hip scores an the UCLA activity scores improve significantly in our stuy. The Harris hip scores associate with complete femoral revision were lower than those following isolate hea-liner exchange. One other stuy showe a significant improvement in the Harris hip scores following hea-liner exchange (from 64.1 points preoperatively to 92.7 points postoperatively), which was comparable with the improvement after acetabular revision (from 66.6 points to 92.1 points) 18. The Harris hip scores in our stuy i not improve as impressively as they i in the stuy by Koh et al. 18, which may be relate to our inclusion of femoral revisions, but our clinical outcomes compare favorably with those in stuies of ifferent types of revision hip arthroplasty (see Appenix). Comparison is somewhat limite by ifferences in proceures an patient populations; however, these ata emonstrate that outcomes are not likely limite by retention of the acetabular component Preoperative Harris hip scores in the current stuy were slightly higher than those in some stuies of other types of revision 24,25, although some patients in our stuy unerwent revision for asymptomatic, extensive osteolysis with a high risk of impening failure an probably ha higher preoperative scores ue to limite pain or ysfunction. As this was a retrospective stuy, we coul not control treatment variables, most notably implant selection. A secon limitation is that this stuy inclue patients in whom a clearly malpositione acetabular component was retaine. Acetabular components outsie the safe zones for anteversion an inclination were assesse on an iniviual basis by the operating surgeon, whose treatment ecisions were base on the risks of bone loss uring revision of a well-fixe acetabular component an subsequent instability or wear from a malpositione component. Another limitation is the lack of analysis of femoral anteversion, as previous reports have suggeste that obtaining a combine femoral an acetabular anteversion of 25 to 50 may be more important for stability than acetabular anteversion alone 31,32. In conclusion, this stuy emonstrate that patient outcomes improve significantly, with a low prevalence of failure requiring repeat revision, at mi-term to long-term follow-up after revision total hip arthroplasty with a retaine acetabular component. This proceure can be safely use in patients with a well-fixe, aequately aligne acetabular component. However, it shoul be performe with caution when the acetabular component position falls outsie of the safe zone for inclination. Highly cross-linke polyethylene is better than conventional polyethylene, so cementing in a highly cross-linke polyethylene liner shoul be consiere if no such liner is commercially available for the existing acetabular component. Longer follow-up is necessary to etermine whether retention of a well-fixe acetabular component is a goo longterm solution.

8 1020 Appenix A table showing a review of the literature on revision total hip arthroplasty outcomes is available with the online version of this article as a ata supplement at jbjs.org. n Muyibat A. Aelani, MD Humaa Nyazee, MPH ohn C. Clohisy, MD Robert L. Barrack, MD Ryan M. Nunley, MD Department of Orthopaeic Surgery, Washington University School of Meicine, 660 South Eucli Avenue, Campus Box 8233, St. Louis, MO aress for M.A. Aelani: muyibat.aelani@gmail.com Nathan A. Mall, MD 6 McBrie & Sons, Center Drive, Suite 204, St. Louis, MO aress: nathanmall@yahoo.com References 1. Clohisy C, Calvert G, Tull F, McDonal D, Maloney W. Reasons for revision hip surgery: a retrospective review. Clin Orthop Relat Res Dec;(429): Maloney W, Waey VM. Management of acetabular bone loss. Instr Course Lect. 2006;55: Lombari AV r, Beren KR. Isolate acetabular liner exchange. Am Aca Orthop Surg May;16(5): Blom AW, Astle L, Loverige, Learmonth ID. 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Clin Orthop Relat Res Feb;468(2): Hallstrom BR, Gollaay G, Vittetoe DA, Harris WH. Cementless acetabular revision with the Harris-Galante porous prosthesis. Results after a minimum of ten years of follow-up. Bone oint Surg Am May;86(5): Lian YY, Yoo MC, Pei FX, Kim KI, Chun SW, Cheng Q. Cementless hemispheric acetabular component for acetabular revision arthroplasty: a 5- to 19-year follow-up stuy. Arthroplasty Apr;23(3): Epub 2007 Oct Sun C, Lian YY, in YH, Zhao CB, Pan SQ, Liu XF. Clinical an raiographic assessment of cementless acetabular revision with morsellise allografts. Int Orthop Dec;33(6): Epub 2009 Feb Lübbeke A, Katz N, Perneger TV, Hoffmeyer P. Primary an revision hip arthroplasty: 5-year outcomes an influence of age an comorbiity. Rheumatol Feb;34(2): Epub 2006 Nov Beaulé PE, Ebramzaeh E, Le Duff M, Prasa R, Amstutz HC. Cementing a liner into a stable cementless acetabular shell: the ouble-socket technique. 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