Hemispheric Press-Fit Components: The Workhorse






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CHAPTER SYNOPSIS


Acetabular revision with uncemented hemispheric components has demonstrated excellent mid- and long-term results. Clinical success and surgeon familiarity with hemisphere cup insertion techniques make this the most common revision technique in North America. Revision applications can use small to jumbo sizes, which are determined according to location and size of bone deficiency. Variable screw hole positions, offsets, constraint, and coatings significantly enhance the utility of these implants.




IMPORTANT POINTS




  • 1

    Preplanning images should accurately define implant position, fixation, and bone defects.


  • 2

    Obtain appropriate modular femoral heads when femoral component retention is planned.


  • 3

    Obtain a comprehensive revision system for both the acetabulum and femur.


  • 4

    Rigid and stable implant fixation is mandatory for success.





CLINICAL/SURGICAL PEARLS




  • 1

    Remove adherent soft tissue from the acetabular fossa to prevent reamer deflection.


  • 2

    Maintain a hemispherical fossa; avoid excessive enlargement.


  • 3

    Medializing slightly can increase peripheral contact and containment.


  • 4

    Compaction (reverse) reaming in soft bone can conserve structure and increase density.


  • 5

    Frequently check reamed fossa visually and with trials; excessive reaming can destroy hemispheric containment.


  • 6

    Rigid fixation often is achieved by wedging the cup between contact points such as the ilium, ischium, and pubis, achieving stability with 50% or less contact.


  • 7

    Supplemental screw fixation is recommended: drill, measure, tap.





CLINICAL/SURGICAL PITFALLS




  • 1

    Overreaming an acetabulum with superolateral deficiency (oval acetabular fossa) can result in anteroposterior column bone loss.


  • 2

    Avoid screw penetration into the sciatic notch.


  • 3

    Avoid screw placement into the posterior-superior and posterior-inferior quadrant safe zones.


  • 4

    Plan for unanticipated defects and inability to gain hemispherical stability (e.g., allograft, cages, reconstruction rings, and cement).





INTRODUCTION


The number of revision total hip arthroplasties is expected to double over the next 25 years. A total of 234,000 primary and 46,000 revision total hip arthroplasties were performed in the United States in 2004. With an estimated hip revision rate of 7% to 14% worldwide, the number of revision total hips performed per year will approach 96,000 in 2030. Common reasons for acetabular revision include aseptic loosening, infection, osteolysis, and instability.


Noncemented hemispherical cups can solve most acetabular revision problems with techniques familiar to experienced hip surgeons. Reinforcement rings, antiprotrusio cages, oblong cups, or custom prostheses are required for some defects and are used as a primary or backup solution as determined by operative planning. Cemented acetabular revision has decreased in popularity in North America because of the high incidence of early radiographic loosening. However, cemented acetabular revision is performed in more than 60% of cases in some European countries, with reports of up to 79% 15-year survivorship.


Successful uncemented acetabular revision requires (1) a supportive, viable bony bed; (2) initial cup stability; and (3) a reliable ingrowth surface. Multiple studies document the mid- and long-term durability of acetabular revision with hemispheric uncemented components. An excellent track record and surgeon familiarity with this technique make it the most popular for acetabular revisions in the United States.




CLASSIFICATION


Preoperative planning for acetabular component revision requires evaluation of the quantity, quality, and location of available bone stock. This information is gleaned from the anteroposterior, lateral, Judet, and false-profile lateral radiographs and, in some cases, computed tomographic scans. The integrity of the anterior and posterior columns is evaluated on the Judet obturator and iliac oblique views, respectively. The false-profile radiograph is a true lateral of the acetabulum and is useful for assessing deficiencies of the anterior dome of the acetabulum, retroacetabular osteolysis, and the posterior column. Acetabular bone deficiency can be classified using the American Academy of Orthopaedic Surgeons (AAOS) or Paprosky classification systems.




INDICATIONS AND CONTRAINDICATIONS


The challenge of uncemented acetabular revision primarily arises from bone loss that can compromise initial fixation and stability and thus the eventual implant osseointegration.


A general consensus in the orthopedic literature exists that an uncemented acetabular shell requires at least 50% host bone contact for initial stability and long-term biologic fixation. Some series have reported higher failure rates when uncemented shells are used to revise major acetabular deficiency such as Paprosky type IIIA and IIIB defects. However, hemispheric acetabular components have been successfully used in cases of severe bone loss and even pelvic discontinuity. This implant’s utility is expanded by the use of various hemispheric design variations such as a jumbo cup; a small cup placed in a high hip center position; a deep profile or protrusio design to increase offset; and manufacture with high-friction cobalt chromium, titanium, or tantalum porous coatings.




MANNER OF USE OF THE HEMISPHERIC SHELL


Jumbo Cup


The jumbo cup has been variously defined as a diameter greater than 65 mm or greater than 61 mm in women or 65 mm in men. It is indicated in cases of severe acetabular bone loss that precludes stable fixation with a standard-sized cup ( Fig. 40-1 ). Whaley et al observed that the jumbo cup increased contact surface area between the implant and the host bone, avoided extensive bone grafting by filling bony defects with the large cup, and translated the hip center laterally and inferiorly to a more anatomic position. Potential disadvantages of the jumbo cup include the inability to restore or preserve bone stock for future revisions with bone grafting because of the space occupied by the large cup. A relative contraindication to jumbo cup use is an acetabulum with severe bone loss and oblong remodeling because of excessive superior migration of the failed acetabular component ( Fig. 40-2 ). In this setting, excessive bone would have to be reamed away to create a hemispherical bed for the jumbo cup, potentially compromising the integrity of the critical anterior and posterior columns.






Figure 40-1


A, Failed loose cemented acetabular component with a Paprosky type IIIA defect. B, Reconstruction with a jumbo cup. Note that the retained femoral component had a fixed 22mm head requiring a special-order 22mm liner.



Figure 40-2


Marked superior migration and oblong remodeling are relative contraindications to jumbo cup use. This case was addressed by using a custom triphlange acetabular component.


The primary goal of hemispherical cup reconstruction is to obtain an initial rigid press fit, which is then augmented by supplementary screw fixation. In the setting of a deficient rim, columns, and medial wall (e.g., Paprosky type III defects), cup–host bone contact of 50% or more may not be possible. However, if perimeter triradiate support is sufficient from remaining bone of the ilium, ischium, and pubis, an initial rigid press fit can be obtained despite acetabular rim, column, and medial deficiencies. In many cases reaming to or slightly through the medial wall can increase the perimeter contact in jumbo cup reconstruction. If an initial rigid perimeter press fit cannot be obtained in this setting of severe bone loss, an alternative technique should be used; this fact cannot be overemphasized.


High Hip Center


Another alternative for managing severe acetabular bone loss is uncemented hemispherical cup placement in a high hip center position. A high hip center has been defined as greater than 30 mm or 35 mm above the inter-teardrop line (Yoder et al and Dearborn and Harris, respectively) or greater than 15 mm superior to the approximate normal femoral head center. Acetabular revision with a high hip center is particularly useful when most of the remaining bone stock is superior to the anatomic hip center. Because host bone contact is possible, complex bone grafting procedures can be avoided. Disadvantages of the high hip center technique include an increased risk of dislocation, possible limb-length shortening, and decreased soft tissue tension. Optimal hip stability and function require adequate soft tissue tension, which can be compromised by a high hip center. Compensatory measures include increased offset or leg length, which can be achieved with modular heads, femoral revision, and protrusio cups.


Bone Grafting


Hemispheric cup fixation and defect reconstruction often require grafting. Morselized autograft or allograft is indicated for cavitary defects. Bulk structural allograft can support a hemispheric cup when large segmental defects preclude primary rigid fixation to host bone. The results of bulk allograft with uncemented fixation are improved when the allograft supports less than 50% of the component. Potential disadvantages of allograft use include graft resorption or fracture with resultant loosening and component migration, which can occur if the graft provides primary structural support.


Protrusion or Lateral Offset


Adjustment of hip soft tissue tension and the anatomic center of rotation can require the use of deep-profile protrusion metal shells or polyethylene inserts, which function to lateralize the femoral head. These implants can be used when standard acetabular placement for significant medial or superomedial acetabular bone defects create instability or in the setting of leg-length discrepancy ( Fig. 40-3 ). These components may be indicated when other techniques to restore soft tissue tension and leg length, such as trochanteric advancement and the use of long, skirted necks, are deemed less desirable or when a well-fixed monoblock femoral component is encountered. A theoretical disadvantage of acetabular components that increase lateral offset is increased transmission of torsion to the bone-component interface with resultant loosening of the component.




Figure 40-3


Lateral offset liners and deep profile/protrusion shells can be used alone or together to adjust offset, soft tissue tension, and stability.

(Image courtesy DePuy Orthopaedics, Warsaw, Ind.)


High-Friction Porous Coating


Newer acetabular components that use high-friction, high-porosity implant coatings such as Trabecular Metal (Zimmer, Warsaw, Ind.), Tritanium (Stryker, Kalamazoo, Mich.), Gription (DePuy, Warsaw, Ind.), and Regenerex (Biomet, Inc., Warsaw, Ind.) may increase the utility of hemispherical components. Early-term success has been reported with Trabecular Metal hemispheric components for pelvic discontinuity and in the irradiated pelvis, an application previously contraindicated for standard implants. However, the results of these techniques are preliminary and should be used with caution until long-term data are available.




Results


Several mid- and long-term acetabular revision series validate the uncemented hemispheric technique ( Table 40-1 ). The most common reason for acetabular revision in these series was aseptic loosening. Adjunctive screw fixation and bone grafting (either cancellous or bulk structural autograft or allograft when indicated) were used in the majority of cases reported and are desirable in most situations. The overall acetabular component repeat revision rate (for any reason) in these series ranged from 0 to 16%. The most common reasons were recurrent dislocation and infection. With repeat revision of hemispherical acetabular components for any reason as an end point, the 10- to 15-year survivorship rates ranged from 81% to 95%. Repeat revision for aseptic loosening was low in all series, ranging from 0 to 9%. Dislocation rates range from 0 to 12% and may be increased after isolated uncemented acetabular revision (with retention of the femoral component). One study found a 20% dislocation rate in isolated acetabular revision cases compared with 8% when both the acetabular and femoral components were revised. This difference was attributed to soft tissue laxity from dissection and capsulectomy required for isolated acetabular exposure.



Table 40-1

Results of Uncemented Hemispheric Cups in Acetabular Revision
























































































Study Year No. of Hips Follow-up (yr) Repeat Revision for any Reason Survivorship
Woolson and Adamson 1996 32 5.8 9% NR
Hallstrom et al 2004 122 12.5 15% 88% at 12 years
Della Valle et al 2005 138 15–19 14% 81% at 15 years
Garcia-Cimbrelo 1999 65 7.4 10.8% 94% at 8 years (Paprosky I and II)40% at 8 years (Paprosky IIIA)57.1% at 8 years (Paprosky IIIB)
Templeton et al 2001 32 12.9 16.4% 100% at 10 years *
Weeden and Paprosky 2006 134 13.2 3.7% NR
Lachiewicz and Poon 1998 57 7 0% NR
Chareancholvanich et al 1999 40 8 13.% 88% at 11.5 years
Jamali et al 2004 63 10.8 14.3% 90.5% at 10 years
Moskal et al 1997 32 4.8 6% NR
Jones and Lachiewicz 2004 211 12 3% 95% at 12 years

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Jan 26, 2019 | Posted by in ORTHOPEDIC | Comments Off on Hemispheric Press-Fit Components: The Workhorse
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