Recommendations for cup containment and impingement may provide conflicting directions for component orientation in total hip arthroplasty. For optimal containment, the cup is positioned with respect to the acetabular bone, resulting in coincidence of the rim of the cup and the acetabulum. This results in good coverage and symmetric load transfer, leading to good long-term stability, but occasionally necessitates more abduction of the cup than that recommended by the safe zone. On the other hand, placement of the cup for an optimal range of motion would lead to only partial containment, with a higher risk of component loosening and revision. The most effective compromise is to use a prosthesis that has a large safe zone, realised by a high head-to-neck ratio, and orienting the cup such that a good containment is achieved and the safe zone is respected. Computer navigation or smart aiming devices may help to find the best relative orientatio

Widmer, K.-H

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REVIEWContainment versus impingement: finding a compromisefor cup placement in total hip arthroplastyK.-H. WidmerPublished online: 28 July 2007# Springer-Verlag 2007Abstract Recommendations for cup containment andimpingement may provide conflicting directions for com-ponent orientation in total hip arthroplasty. For optimalcontainment, the cup is positioned with respect to theacetabular bone, resulting in coincidence of the rim of thecup and the acetabulum. This results in good coverage andsymmetric load transfer, leading to good long-term stability,but occasionally necessitates more abduction of the cupthan that recommended by the safe zone. On the otherhand, placement of the cup for an optimal range of motionwould lead to only partial containment, with a higher riskof component loosening and revision. The most effectivecompromise is to use a prosthesis that has a large safe zone,realised by a high head-to-neck ratio, and orienting the cupsuch that a good containment is achieved and the safe zoneis respected. Computer navigation or smart aiming devicesmay help to find the best relative orientation.IntroductionBesides pain relief, patients expect hip replacement torestore good joint function with successful long-termfixation of the components. There is no doubt that a highstandard in total hip replacement has been achievednowadays and nearly one million patients are providedwith new perspectives in their lives from this developmentworld-wide each year. But what is necessary to realisesecure component fixation and orientation, especially in thenon-cemented cup?Cementless pressfit acetabular cups have been widelyaccepted and became the standard in total hip arthroplasty.The fixation of these cups in acetabular bone and their loadtransfer has been studied by several groups [1]. There isalso a trend to prefer hemispheric cups that are ideallysuited for pressfit techniques and to preserve as muchacetabular bone as possible. These cups distribute com-pressive fixation forces to the periphery near the equator ofthe acetabulum, where they generate friction forces, whichhold the cup in place [2–4]. Such a load pattern is close tothat in a natural hip joint. Therefore, it can be hypothesisedthat this load transfer guarantees the best long-term fixationof an acetabular cup, as it does not alter the normal loadtransfer of a natural hip joint [5].Furthermore, the cups must be implanted in such a waythat the best containment is achieved, i.e. the rim of theacetabular bone and the rim of the socket should coincideas closely as possible. This may lead to a higher cupabduction that may be different from cup orientations thatare recommended with respect to a good range of motion.In addition, it may generate prosthetic impingement,leading to additional problems, such as subluxation,dislocation and rim damage in hard-on-hard bearings. Thequestion arises on how this conflict can be resolved. Thispaper addresses this conflict and tries to give possiblesolutions.Cup containmentThere are numerous recommendations on how to orient theprosthetic cup with respect to the rim of the acetabularbone. These cups are evaluated as fully contained and lessInternational Orthopaedics (SICOT) (2007) 31 (Suppl 1):S29–S33DOI 10.1007/s00264-007-0429-3In the preparation of this article, no funding was received in any formwhatever. The content of this manuscript is not linked to any productthat is commercially available and there are no conflicts of interestdirectly or indirectly relevant to the contents of this manuscript.K.-H. Widmer (*)Department for Orthopedic Surgery and Traumatology,Kantonsspital Schaffhausen,8208 Schaffhausen, Switzerlande-mail: Karl-Heinz.Widmer@unibas.chradiolucencies were seen in the interface of these cups [6].A more horizontal orientation leads to a partial containmentonly and a higher rate of radiolucencies is seen thereafter.Furthermore, the lack of containment was correlated with ahigher rate of loosening in the long term [7].Good containment of the cup is necessary for a reliablepressfit. The pressfit technique requires that contained partson the outer surface of the cup be opposed in order to buildup opposing forces in the implant–bone interface thatcompress the prosthetic socket. Such compressing forcesare provided at those parts of the acetabulum that aresupported by the iliac, pubic and ischial bones [1] (Fig. 1).The cup should be supported by the cortical bone on itsouter surface, i.e. the rim of the cup should not sink into thereamed acetabulum, otherwise, the cup is supported bycancellous bone only. Cancellous support alone weakensprimary stability and will disturb osseointegration and,finally, completely prevent it (Fig. 2). So, cortical contact ofthe outer surface of the cup is an essential feature of pressfitimplantation.ImpingementImpingement may occur as component-to-component im-pingement, bone-to-bone impingement or component-to-bone impingement. If any scar tissue is formed aroundthe neck, this tissue can also interfere with the prosthesisneck, leading to additional soft tissue impingement. Whilebone-to-bone impingement is addressed by the offset of theprosthesis, component-to-component impingement is high-ly dependent on the design parameters and componentorientation [8, 9].There are numerous papers covering the recommenda-tions on how the components should be positioned in totalhip arthroplasty. Lewinnek introduced the safe zoneconcept in 1978 [10]. He derived his recommendationsfrom clinical findings: cup orientation from 5° to 25° ofanteversion and 30° to 50° of abduction showed a lowerrate of dislocation. Nothing is said about the stemorientation. Apparently, the cup and neck should beoriented relative to each other, accordingly, in order toprevent cup-to-neck impingement. This is expressed by theso-called “combined version” for the cup and stem.Ranawat recommends different combined versions formen and women [11], while computer simulation withthree-dimensional models revealed that the size of the safezone is optimised when the “intended range of motion”concept is introduced. This concept tells the surgeon whichcomponent orientation he/she should adhere to when apredefined range of motion should be reached by thepatient [12]. There is a linear correlation between cupanteversion and stem antetorsion [12], and also betweencup abduction and the neck-to-shaft angle of straight stems[8]. The optimal range for the neck-to-shaft angle isbetween 125° and 130°. These stems allow the lowest cupabductions, which is beneficial for a small edge loading andfor a reduced risk of dislocation. In addition, it is close tothe CCD angles found in the normal femur [13–15].The amazing thing about component orientation in totalhip arthroplasty is that there is no need to stick to anabsolute orientation when implanting the first component,but, as soon as the position of this first component is fixed,the second component has to be positioned in a compliantorientation to the first component. Hence, the relativepositioning of the components is more important than theabsolute positioning with regard to bony landmarks.This goal is best achieved when the second component isintentionally oriented relative to the first component, i.e. thesurgeon may follow the “stem first” concept, where the cupis positioned relative to the stem after the trial stem hasbeen implanted [12]. As a matter of course, the orientationof the cup relative to the stem has to be known for eachprosthesis system when this concept is applied. Alterna-tively, one may introduce modularity in cups or stems toaccommodate for relative component positioning.The most efficient way to reduce the risk of impinge-ment is increasing the head-to-neck ratio by eitherincreasing the head size or by reducing the neck diameterFig. 1 Most of the load istransferred at these three dis-tinct areas, where the acetabularbone is supported by the iliac,pubic and ischial bone (three-spot support, left acetabulum)Fig. 2 Seating the cup beyond the cortical rim of the acetabulum willput its osseointegration at risk. Cortical rim contact of the cup isnecessary for primary stability and secure osseointegrationS30 International Orthopaedics (SICOT) (2007) 31 (Suppl 1):S29–S33or both. The head-to-neck ratio should at least be 2 or more,i.e. the diameter of the head should be double the neckdiameter. Increasing the head-to-neck ratio will increase thesize of the safe zone (Fig. 3). As one would expect, saidchanges in head size have a greater influence on the safezone in smaller heads (e.g. 22–26 mm) than in larger heads(e.g. 38–42 mm).Additionally, the risk of dislocation is reduced, not onlyby preventing impingement, but also by increasing thedistance that is required to lift the head out of the cup. Soft-tissue tension increases when the prosthesis head is liftedand this tension counteracts further dislocation. Fortunately,larger head sizes are a valuable option nowadays, incombination with cross-linked polyethylene and hard-on-hard bearings (e.g. ceramic-on-ceramic and metal-on-metal), which provide enhanced wear resistance, withoutthe need for excessive reaming of the acetabulum.The simulations for the range of motion suggestorienting the cup to be less inclined and more antevertedin prosthesis systems with a straight stem [8]. Combininglow abduction and low anteversion is not recommended, asit leads to impingement during flexion. Moreover, oneshould not stick to fixed absolute values defining theorientation of the components, but one should position thecomponents with regard to the specific safe zones that canbe determined for each prosthesis system (Fig. 4). Obvi-ously, there is a strong interrelationship between the fourangles defining the orientation of both components. Neck-shaft angle and antetorsion of the stem greatly determinehow the cup should be positioned. When offset stems areused with a reduced neck-shaft angle, the anteversion of thecup should be reduced too [8]. Neck-shaft angles greaterthan 135° lead to a dramatic reduction in the range ofmotion and should be avoided.DiscussionThe orientation of the natural acetabular opening in thehuman pelvis can vary from anteversion to retroversion,and its abduction can vary from lower to higher degreestoo. Furthermore, the cavity tends to be shallow in cases ofdysplasia and rather deep in cases with protrusion of thefemoral head. Hence, orienting the prosthetic cup withregard to the acetabular rim alone may result in a widespectrum of cup orientations. However, in each case, theimproved containment of the cup is expected to lead togreater long-term stability of the prosthesis. One may evenaccept extreme but compliant positions of the componentsin order to obtain good component fixation and long-termstability, as long as the morphology of the proximal femurand the pelvis allows the stem and the cup to be placed incomplementary orientations.The situation becomes more challenging during prepa-ration of the acetabulum or the femur, when it becomesclear that it is infeasible to orient the components incomplementary positions. This happens quite frequently,because, in general, the orientation of the neck is notreflected by a corresponding orientation of the acetabulumand, moreover, the abduction of the acetabulum is higherFig. 3 The safe zone for cuporientation increases with in-creased head-to-neck ratio, pro-viding more room for errorInternational Orthopaedics (SICOT) (2007) 31 (Suppl 1):S29–S33 S31than the recommended abduction for the prosthetic socket.So, it is inescapable to find a compromise between thecontainment of the cup and the risk of impingement. If everin doubt, good containment is preferable in order to achievea good fixation of both components, because long-termfixation of the components is an indispensable requirementfor a successful total hip arthroplasty. The diagramsshowing the safe zones may help to find a goodcompromise. For example, they indicate that a smallanteversion or even retroversion of the acetabulum can bebest met with a stem having a low neck-to-shaft angle, asthese stems complement the placement of the acetabularcups in less anteversion.In cases with underdeveloped acetabular bone stock, likein a case of congenital dysplastic hip, one should aim forthe optimal relative component fixation (Fig. 5). Here,reaming the acetabular bone according to the basic require-ments of good cup fixation at the correct site helps torestore the biomechanics of the hip. Sometimes, it isadditionally helpful to use stems that allow the prostheticneck to be placed in a wide range of rotational positionsindependent of the original morphology of the proximalfemur.It may also be suggested that the relative orientation ofthe outer surface of the cup and the articulating surface ofthe liner should be decoupled to allow differences inorientation. With such a modular cup, the outer surfacecan be oriented in full accordance with the needs of optimalcup fixation and containment, whereas the inner articulatingsurface can be oriented according to the recommendationfor a maximised safe zone providing the greatest range ofmotion, together with a given stem.What is the role of computer-assisted surgery in total hiparthroplasty, in particular, computer navigation? Above all,computer navigation software should have a built-indatabase that illustrates the safe zone for each prosthesissystem considering its specific design and implantationparameters. Based on this built-in knowledge, such a navi-gation system converts from a simple three-dimensionalmeasuring system to a real navigation tool that supportsthe surgeon in finding the best way of componentorientation, especially in setting the complementary orien-tation of the cup or the femoral stem when the orientationFig. 5 Secure fixation is achieved by precise reaming of theacetabular bone, even in dysplastic pelvises. Hip biomechanics isrestored on the right side after distraction (distance=8.5 cm)Fig. 4 Shape and position ofthe safe zone is dependent onthe neck-to-shaft angle of thestemS32 International Orthopaedics (SICOT) (2007) 31 (Suppl 1):S29–S33of its mating component has already been set. In followingsuch a procedure, slight misplacement of the first compo-nent does not have any adverse effects, as the navigationsystem can recompute the best orientation of the secondcomponent to restore the relative orientation of bothcomponents to the safe zone. In particular, there is nolonger any need to stick to fixed, predefined values for thepositioning of either component.There is also a role for smart non-computer-basedaiming devices that can be constructed to provide system-specific guidance in ensuring that components are orientedwithin the safe zone of each implant system. Although sucha device may lack the versatility of a real computer-basednavigation system, it may provide enough support bysolving the task of orienting the components without theneed for the time-consuming setup and application of aconventional navigation system. Such an option could behighly attractive for a modular cup or a modular stem.References1. Elke R, Marugg S (1992) Transmission of force to the trabecularstructures of the proximal end of the femur (in German).Orthopäde 21:51–562. Laursen JO, Petersen B, Mossing NB (1998) The Richards Series2 total hip prosthesis: a 13-year study and radiographic evaluation.Orthopedics 21:277–2823. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmermann JR(1978) Dislocations after total hip-replacement arthroplasties. JBone Joint Surg Am 60(2):217–2204. Morscher EW (1992) Current status of acetabular fixation inprimary total hip arthroplasty. Clin Orthop Relat Res 274:172–1935. Massin P, Geais L, Astoin E, Simondi M, Lavaste F (2000) Theanatomic basis for the concept of lateralized femoral stems: afrontal plane radiographic study of the proximal femur. JArthroplasty 15:93–1016. Noble PC, Alexander JW, Lindahl LJ, Yew DT, Granberry WM,Tullos HS (1988) The anatomic basis of femoral componentdesign. Clin Orthop Relat Res 235:148–1657. Ranawat CS, Maynard MJ (1991) Modern techniques of cementedtotal hip arthroplasty. Techniques Orthoped 6:17–258. Sarmiento A, Ebramzadeh E, Gogan WJ, McKellop HA (1990)Cup containment and orientation in cemented total hip arthro-plasties. J Bone Joint Surg Br 72:996–10029. Schidlo C, Becker C, Jansson V, Refior J (1999) Change in theCCD angle and the femoral anteversion angle by hip prosthesisimplantation (in German). Z Orthop Ihre Grenzgeb 137:259–26410. Shirazi-Adl A, Dammak M, Paiement G (1997) Experimentaldetermination of friction characteristics at the trabecular bone/porous-coated metal interface in cementless implants. J BiomedMater Res 27:167–17511. Widmer K-H, Zurfluh B (2004) Compliant positioning of total hipcomponents for optimal range of motion. J Orthop Res 22:815–82112. Widmer K-H, Majewski M (2005) The impact of the CCD-angleon range of motion and cup positioning in total hip arthroplasty.Clin Biomech 20:723–72813. Widmer K-H, Zurfluh B, Morscher EW (1997) Contact surfaceand pressure load at implant–bone interface in press-fit cupscompared to natural hip joints (in German). Orthopäde 26:181–18914. Widmer K-H, Zurfluh B, Morscher EW (2002) Load transfer andfixation mode of press-fit acetabular sockets. J Arthroplasty17:926–93515. Yoshimine F (2006) The safe-zones for combined cup and neckanteversions that fulfill the essential range of motion and theiroptimum combination in total hip replacements. J Biomech39:1315–1323International Orthopaedics (SICOT) (2007) 31 (Suppl 1):S29–S33 S33

Containment versus impingement: finding a compromise for cup placement in total hip arthroplasty

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Containment versus impingement: finding a compromise for cup placement in total hip arthroplasty

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