Cementless Acetabular Cup Technique

Introduction

• Use of cementless fixation in total hip arthroplasty permits direct osteointegration of the implant with long-term biologic fixation to bone.

• Problems with modular cementless acetabular components have been related to wear or mechanical failure of ultrahigh-molecular-weight polyethylene (UHMWPE) components sterilized by gamma irradiation in air, rather than to loosening (Clohisy and Harris, 1999; Dorr et al., 1998).

   

  P ITFALLS

  • Initial stability of an uncemented acetabular component can be achieved with use of screws, spikes, peripheral fins, or a press-fit in which the diameter of the acetabular cup is slightly larger than the reamed acetabulum. Press-fitting requires adequate acetabular bone stock. If bone stock is poor, use of a cementless acetabular component is still appropriate, but additional screw fixation is often necessary.

   

• The hemispherical shape of the acetabular cavity permits very good initial stability of the acetabular component and a large surface area for bone ingrowth. Once initial stability is achieved, long-term biologic fixation occurs reliably.

• Modular components also permit intraoperative optimization in implant positioning to maximize range of motion to impingement and stability, and use of liners with different head diameters, constraint, and offset.

  Controversies

  • Poor vascularity of the periarticular bone associated with radiation osteonecrosis of the pelvis may be considered a relative contraindication to cementless fixation, although cement fixation has not been associated with favorable results in this patient population either. Porous ingrowth surfaces, which provide greater initial friction between the implant and bone, may provide more reliable results with cementless fixation (Rose et al., 2006).

  • Cement fixation has been advocated for use in Paget’s disease, although very good results have been achieved with cementless acetabular components (Hozack et al., 1999).

   

• Improvements in UHMWPE sterilization and in alternative bearing surface materials, including highly cross-linked UHMWPE, ceramic-on-ceramic, and metal-on-metal bearings, should further improve the longevity of modular cementless acetabular components in total hip arthroplasty.

  Indications

• Cementless acetabular cups are indicated for treatment of symptomatic osteoarthritis, posttraumatic arthritis, or inflammatory arthritis of the hip requiring total hip arthroplasty in which the acetabular bone stock is sufficient to provide mechanical support for initial stability of an uncemented acetabular component.

   

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  Examination/Imaging

  • Acetabular anatomy is best visualized on the anteroposterior pelvic radiograph. Figure 1A shows an arthritic hip in a 74-year-old man with retained hardware in the femoral head. The outline of the teardrop and acetabular joint surface is illustrated in Figure 1B. Figure 2 illustrates the amount of lateralization of the femoral head from the floor of the acetabulum. The left arrow shows the lateral border of the teardrop, which represents the anatomic floor of the acetabulum, and the right arrow represents the lateralized base of the arthritic acetabulum.

 

 

 

A B

FIGURE 1

 

 

 

FIGURE 2

 

 

 

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Treatment Options

• Treatment options include press-fit fixation, screw fixation, and supplemental spikes or fins.

• Press-fit fixation with a cup that is slightly larger than the reamed acetabular cavity requires both adequate peripheral coverage of the acetabular component rim and adequate bone quality. Figure 4 shows arrows that indicate that pressure from the bone against the peripheral rim of the acetabular component produces a force vector that helps to stabilize the implant while pressure against the more medial portions of the cup provides less stability.

• If the bony rim is deficient, such as that which typically occurs in dysplasia, or the bone is osteoporotic, then additional screw fixation is likely to be necessary.

• Acetabular components with supplemental spike or fin fixation do not require press-fitting or use of screws, but if the bone is sclerotic, then full seating of the acetabular component such as that shown in Figure 5 (note arrows) may not be possible if the spikes or fins do not fully penetrate into the hard bone.

 

FIGURE 3

 

Cementless Acetabular Cup Technique

 

• Figure 3 shows an overlay template of the acetabular component, which is medialized to the acetabular floor and placed at 40–45° of abduction. If there is adequate lateral bone coverage of the acetabular component, as shown in this case, then peripheral coverage of the rim should be sufficient to permit press-fit stability.

   

   

   

  FIGURE 4

   

   

   

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  Cementless Acetabular Cup Technique

   

  FIGURE 5

   

  Surgical Anatomy

  • Figure 6A shows the floor of the acetabulum, which is a reliable anatomic landmark and should be identified prior to reaming. The inner circular line in Figure 6B outlines the acetabular floor, and the acetabular rim is shown by the outer circular line. The acetabular floor may be covered with soft tissue or osteophytes. This represents the lateral border of the teardrop seen radiographically.

 

 

 

A B C

FIGURE 6

 

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• The acetabular labrum should be débrided and the bony rim identified circumferentially.

• The sciatic nerve is posterior to the piriformis tendon and traverses over the surface of the ischium. It should be identified by palpation and protected during placement of acetabular retractors. The nerve is under tension during hip flexion. The hip should not be fully flexed during acetabular reaming because the posterior retractor is adjacent to the sciatic nerve and may cause nerve compression (Satcher et al., 2003). Figure 6C shows that the sciatic nerve (arrows) can be in close proximity to the posterior acetabular retractor when the hip is flexed.

  Positioning

• For either a posterior or direct lateral (Hardinge) approach, the patient is positioned in the lateral decibitus position.

   

   

   

  P EARLS

  • An axillary roll should be placed to protect the contralateral brachial plexus.

     

  • The fibular head of the contralateral knee should be padded to prevent peroneal nerve compression.

     

  • The contralateral hip should be flexed slightly since a flexion contracture could affect pelvic position.

     

  • The ipsilateral arm should be placed on an upper armboard (Fig. 7A) or pillows so that it is parallel to the lower arm to avoid rotation of the upper spine.

     

    P ITFALLS

  • No positioning device will hold the pelvis in a completely rigid position during surgery. Pelvic position should be checked periodically during the procedure by palpation of the anterior superior iliac spine (ASIS). Pelvic stability is reduced more in obese patients.

     

  • If a posterior approach is used, the femur and lower leg are retracted anteriorly during acetabular preparation. The weight of the thigh and lower leg can cause the ilium to tilt forward, which may lead to insufficient anteversion of the acetabular component. Either the pelvic position should be re-established during acetabular cup placement by pulling the ilium posteriorly or the acetabular insertion device flexed to compensate for anterior tilt of the ilium.

   

   

   

   

   

  Equipment

  • An inflatable beanbag with or without additional side supports or various devices with vertical padded posts can be used to secure the patient in the lateral decubitis position. Figure 7B shows a patient positioned using a padded post against the sacrum. Two other padded posts are placed against the pubic symphysis and ASIS to secure the patient in the lateral decubitis position. For very obese patients, it may be necessary to tape the thorax to the operating room table in order to provide more support.

   

  A

   

  165

   

   

   

  Controversies

  • Vertical padded post devices place pressure over bony prominences, including the pubic symphysis, ASIS, and sacrum. This provides more stability than an inflatable beanbag, but pressure over the ASIS can occasionally cause sensory disturbances in the lateral femoral cutaneous nerve (Kitson and Ashworth, 2002).

   

  Cementless Acetabular Cup Technique

   

  B

   

  FIGURE 7

   

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  Instrumentation

  • A self-retaining, Charnley-type retractor is used to retract the skin, subcutaneous tissue, and fascial layers. Figure 8A shows Holman or similar retractors that have been placed over the anterior and posteroinferior acetabular rims to lever the proximal femur and soft tissues. The Charnley (C), posterior Holman (P), and anterior Holman (A) retractors, and proximal femur (F) are labeled in Figure 8B.

   

  Cementless Acetabular Cup Technique

   

  Portals/Exposures

• The hip can be exposed through a posterior, direct lateral (Hardinge), or anterior approach.

 

 

P EARLS

• The acetabulum should be exposed circumferentially.

• The acetabular labrum is débrided to identify the bony rim.

   

• For a posterior approach, the femur is retracted anteriorly, and for lateral or anterior approaches, the femur is retracted posteriorly.

   

  P ITFALLS

• Adequate mobilization of the proximal femur is necessary to expose the acetabulum.

   

• The inferior hip capsule may restrict adequate mobilization of the femur.

   

• Circumferential capsulotomy or capsulectomy will provide better mobilization of the femur and acetabular exposure.

Controversies

• Minimally invasive approaches, which retain more of the capsule and short external rotator tendon attachments to the proximal femur, may permit better hip stability and earlier return of function after surgery. However, these approaches may not be suitable in heavy patients or those with stiff hips, in

whom more soft tissue dissection is needed to mobilize the femur and permit adequate acetabular exposure.

 

 

 

 

A B

FIGURE 8

 

 

P ITFALLS

• Inadequate exposure can lead to asymmetric reaming. With a posterior approach, inadequate anterior mobilization or retraction of the proximal femur can lead to impingement of the femur against the acetabular reamer, which causes inadvertent reaming through the posterior acetabular wall (the opposite is the case with direct lateral or anterior approaches).

• The more commonly used reamers (sizes 50 to 56 mm) may become dull from repeated use and require replacement. Sharpening of used reamers can reduce the reamer diameter.

 

Procedure

Step 1: Acetabular Reaming

• After the medial floor of the acetabulum is identified, a relatively small reamer is used to ream medially to the level of the acetabular floor. Figure 9 shows that the acetabular floor is in continuity with the reamed acetabulum after medialization. The reamed cavity will act to centralize successive reamers that are used to expand the acetabulum.

• Sequential reamers, usually in 2-mm increments, are used to expand the size of the reamed acetabulum to the peripheral rim, while maintaining the integrity of the medial and lateral acetabular walls. Figure 10 shows that the floor is still seen and the acetabular walls are preserved after final reaming.

 

 

 

 

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Instrumentation/ Implantation

• Hemispherical “cheese grater” reamers are typically used to prepare the acetabulum. Smaller “cutout” reamers may be easier to insert through small incisions but also can catch on the edge of the acetabular rim during reaming.

• A trial shell that is the same diameter as the largest reamer used should be inserted to confirm that the reamed acetabular cavity is hemispherical and the same diameter as the final reamer size.

 

Cementless Acetabular Cup Technique

 

FIGURE 9

 

 

 

 

FIGURE 10

 

Controversies

• Medialization to the acetabular floor may not always be necessary. Medialization permits better peripheral coverage of the acetabular component, but also removes more bone than incomplete medialization. Positioning the cup in a more lateral location retains more medial bone, which may be beneficial if later cup removal and revision are needed. However, lateral placement of the center of the hip also increases joint reactive forces, which may contribute to increased wear of the bearing surface.

 

 

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P EARLS

• Mechanical alignment devices are helpful to orient the acetabular component. The vertical limb of most alignment devices is oriented at 45°, so this should be slightly more abducted than a vertical position to provide 40° of abduction (Fig. 11A). The horizontal limb of the alignment device, shown as an X-shaped bar in Figure 11B, is oriented in line with the axis of the upper body to orient the component in 20–25° of anteversion.

   

  P ITFALLS

• The pelvis may tilt forward during acetabular preparation and cup insertion. Pelvic position should be corrected or the acetabular cup flexed forward during insertion to compensate for any forward tilt of the ilium.

 

Cementless Acetabular Cup Technique

 

Step 2: Alignment

• The acetabular component should be oriented at approximately 40–45° of abduction and 20–25° of anteversion in order to maximize stability and range of motion to impingement.

• Once the acetabular cup is inserted, a trial liner should be used. After the femoral component is inserted, range of motion to impingement and stability are assessed. If impingement occurs, then the cup position should be changed if necessary.

   

   

   

  Instrumentation

  • Alignment devices usually contain a vertical positioning bar at 45° of abduction and also an anteversion guide. However, the position of the insertion device rod alone can be used and its position also assessed during cup placement.

   

  A

  FIGURE 11

   

  Controversies

  • Compared to mechanical alignment devices, computer templating permits more accurate acetabular cup placement, but requires additional surgical time, equipment, and staff and surgeon training. The relative benefit of computer templating may not outweigh the expense and the exposure of the patient to additional surgical time. However, further developments in computer templating with less costly and time-consuming techniques may permit more routine use of this technology.

   

   

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  Cementless Acetabular Cup Technique

   

  B

  FIGURE 11, cont’d

   

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  P ITFALLS

  • If a constant amount of acetabular cup oversizing is selected, such as 2 mm for all acetabuli, the relative percent expansion of the acetabulum that occurs during insertion of the cup will be greater in a small compared to a large acetabulum (Ries and Harbaugh, 1997). This results in a greater risk of fracture in a small compared to a large acetabulum unless a smaller amount of oversizing is used (1 mm in small acetabuli and 2 mm in larger acetabuli).

   

  Cementless Acetabular Cup Technique

   

  Step 3: Insertion of a Press-Fit Acetabular Component

• For press-fit fixation, the acetabular component is slightly larger than the reamed acetabular cavity.

• Stability of the acetabular component is achieved from pressure of the peripheral acetabular bone against the rim of the component, while pressure against the more medial portions of the cup produces a force vector against the cup, which tends to provide less stability (see Fig. 4).

• After insertion of the metal shell, peripheral osteophytes (arrows in Fig. 12A) are removed with an osteotome or rongeur to prevent impingement.

  Instrumentation/ Implantation

  • The acetabular cup should be aligned in the desired position of abduction and anteversion prior to impacting and final seating of the cup. If a change in cup position is needed, the implant should be removed completely and reoriented prior to reseating.

   

• Figure 12B shows the postoperative position of a press-fit acetabular cup. The acetabular component has been medialized to the teardrop or acetabular floor, and there is adequate lateral coverage to permit stable press-fit stability.

 

Controversies

• Press-fit fixation avoids use of screws or cups with screw holes, but also results in a risk of acetabular fracture or inadequate fixation. Excellent clinical results have been achieved with cups having screws, so the need for press-fit fixation is not clear. However, use of an implant without screw holes provides a large surface area for support of the liner, limits access of debris through screw holes, and is necessary for large-diameter metal-on-metal resurfacing arthroplasty.

 

 

 

 

A B

FIGURE 12

 

 

P EARLS

• Screws should be directed superiorly and posteriorly. Trauma to the iliac vessels can occur from anterior drill insertion.

   

  P ITFALLS

• The iliac veins lie along the inner pelvis opposite the anterior medial wall of the acetabulum. Trauma to the iliac vessels can occur from anterior drill insertion. The anterior superior quadrant of the acetabulum should be avoided during screw placement.

 

Cementless Acetabular Cup Technique

 

Step 4: Screw Fixation

• Figure 13A shows a long (40- to 50-mm) intramedullary iliac screw, which has been placed superiorly and does not exit through the cortex of the ilium. A short (20- to 25-mm) screw has been placed posteriorly toward the sciatic notch, which exits through the outer table of the pelvis.

• The most commonly used cup sizes include two rings of screw holes, such as those shown in Figure 13B. The long superior iliac screw is placed through a screw hole in the inner ring while the posterior screw may be inserted through either the inner or outer ring screw holes (Fig. 13C and 13D).

 

 

 

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Instrumentation/ Implantation

• Flexible drill bits and screwdrivers are necessary to insert screws through the dome of the acetabular cup. Both the drill and screwdriver should be inserted with the handle as horizontal as possible to maximize torque during insertion. The distal end of the incision limits how horizontal the drill or screwdriver can be placed.

 

A

 

 

 

Controversies

• Screw fixation can be achieved with either dome or peripheral screws. Peripheral screws require a cup with a large enough metallic rim to provide circumferential metal around the screw head, which can limit the inner diameter of the cup and size of the liner used.

 

B

FIGURE 13

 

C

D

 

 

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FIGURE 13, cont’d

 

 

Controversies

• Weight-bearing stresses can cause torque on the acetabular component, which could lead to early displacement, particularly with a press-fit technique in relatively poor bone stock. However, displacement of an acetabular component in the early postoperative period likely requires trauma such as would occur from a fall. Even with full weight-bearing activity permitted, use of some type of ambulatory support (walker or crutches) is appropriate to minimize the risk of a postoperative fall.

 

 

 

P EARLS

• If postoperative radiation therapy is required to minimize the risk of heterotopic bone formation, the acetabular component bone-implant interface should be shielded to permit bone ingrowth into the surface of the component.

   

  P ITFALLS

• An anteroposterior radiograph should be obtained in the recovery room after surgery to ensure adequate reduction of the hip and component placement.

   

• Dislocation may occur while the patient is under anesthesia, during transfers, or during positioning in the recovery room, particularly if long-acting spinal anesthetics are used, which result in decreased muscle tone.

 

Postoperative Care and Expected Outcomes

• Hip dislocation precautions should be followed.

• Weight bearing does not necessarily need to be restricted. Weight-bearing restriction is typically required for use of an uncemented femoral component, which can subside, but the acetabular component is inherently stable because of the hemispherical geometry of the acetabular cavity, and can tolerate early weight-bearing stresses.

 

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Evidence

Clohisy JC, Harris WH. The Harris-Galante porous-coated acetabular component with screw fixation: an average ten-year follow-up study. J Bone Joint Surg [Am].

1999;81:66–73.

 

One hundred seventy-seven patients (196 hips) with cementless porous coated Harris Galante acetabular components were reviewed after an average follow-up of 122 months. Eight well fixed acetabular shells (4 percent) were revised: three were revised because of dissociation of the liner, three were revised during revision of the femoral component, and two were revised because of retroacetabular osteolysis. No acetabular component migrated, was radiographically loose, or was revised because of aseptic loosening.

 

Dorr LD, Wan Z, Cohen J. Hemispheric titanium porous coated acetabular component without screw fixation. Clin Orthop Relat Res. 1998;(351):158–68.

 

One hundred eight patients (115 hips) with cementless anatomic porous replacement hemispheric acetabular components implanted without screw fixation were evaluated after an average follow-up of 6 years. No acetabular metal shell had been revised for loosening or was radiographically loose. Reoperation was required in nine (8 percent) hips because of polyethylene insert wear or dissociation.

 

Hozack WJ, Rushton SA, Carey C, Sakalkale D, Rothman RH. Uncemented total hip arthroplasty in Pagets disease of the hip. J Arthroplasty. 1999;14:872–6.

 

Five patients with Paget’s disease involving the acetabulum were treated with cementless acetabular components during total hip arthroplasty. At an average followup of 5.8 years, all acetabular components were well fixed radiographically with no migration or loosening.

 

Kitson J, Ashworth MJ: Meralgia paraesthetica: a complication of patient positioning device in total hip replacement. J Bone Joint Surg [Br]. 2002;84:589–90.

 

The authors described three patients who developed meralgia paraesthetica in association with the use of padded post positioning devices during total hip arthroplasty.

 

Ries MD, Harbaugh M. Acetabular strains produced by oversized press fit cups. Clin Orthop Relat Res. 1997;(334):276–81.

 

Using finite element analysis, the authors found that with a constant amount of oversizing (for example, a cup which is one mm larger than the reamed acetabulum for all acetabular sizes) the relative expansion of the acetabulum and risk of fracture during press fitting will be greater in a small compared to a large acetabulum.

 

Rose PS, Halasy M, Hanssen AD, Sim FH, Lewallen DG, Berry DJ. Total Hip Arthroplasty After Pelvic Radiation: Results with Trabecular Metal Acetabular Components.

Washington, DC: American Academy of Orthopedic Surgeons, 2006.

 

Twelve patients (13 hips) with previous pelvic irradiation therapy were treated with cementless acetabular reconstruction using trabecular metal components. After an average follow-up of 29 months none of the components had migrated or loosened.

 

Satcher R, Noss RS, Yingling C, Ressler J, Ries MD. The use of motor evoked potentials to monitor sciatic nerve status during revision total hip arthroplasty. J Arthroplasty. 2003;18:329–32.

 

Motor-evoked potentials (MEPs) were used in combination with electromyography (EMG) monitoring during revision total hip arthroplasty in 27 patients. Significant electrical events occurred, most commonly during acetabular retraction while the hip was in a flexed