Prosthesis Selection in Hip Revision Surgery: Femoral Prostheses (Part II)

lPaprosky Type I Defects

The Paprosky classification, introduced in 2003, is widely used due to its guidance on revision prosthesis selection. Initially based on Paprosky's use of fully-coated cylindrical cobalt-chromium prostheses, prosthesis selection has since evolved, phasing out cobalt-chromium stems. The classification's theoretical basis is to ensure the prosthesis bypasses poor bone quality, utilizing distal fixation. Preoperative X-rays can sometimes underestimate the extent of bone loss, so surgeons should be prepared for more severe bone defects.

In Paprosky Type I defects, there is minimal cancellous bone loss in the metaphysis, with the diaphysis remaining intact. This is common in revision cases of biologic femoral prostheses without bone ingrowth. Cemented or proximally/distally fixed uncemented prostheses can be used for revision. However, outcomes using cemented or proximally fixed prostheses in Type I defects are not ideal. If a cemented prosthesis is chosen, small drills should be used to thoroughly roughen the proximal femoral canal to enhance the bond between the cement and bone.

l Paprosky Type II Defects

Type II defects exhibit extensive cancellous bone loss in the metaphysis while maintaining an intact diaphysis. This is the most common scenario in femoral revisions, typically seen with early loosening of both cemented and biologic prostheses (Figure 1).

Figure 1: Paprosky Type II defect shows extensive proximal bone loss with an intact femoral diaphysis, common in early loosening of non-cemented prostheses.

Using cemented prostheses is not recommended for these cases due to the smooth cortical surface left by the loss of cancellous bone in the metaphysis, making cement anchorage difficult. Revision with cemented prostheses faces high failure rates, especially if the initial prosthesis was biologic. Despite improvements in cement techniques, survival rates of cemented prostheses in revisions remain lower than biologic prostheses.

Proximally coated, distally fixed biologic prostheses, modular or monoblock tapered stems, and extensively coated cylindrical stems are commonly used for revisions in these cases. Preoperative assessment of femoral remodeling is crucial. If significant remodeling is present, an extended trochanteric osteotomy (ETO) may be necessary, with distal fixation using cylindrical or tapered stems (Figure 2 A-B).

Figure 2 A-B: Following ETO, cylindrical (A) or tapered (B) distally fixed femoral prostheses should be used.

Fully coated cylindrical stems have a higher dislocation rate due to the requirement for proximal bone support, potentially leading to posterior tilting and joint instability during placement. They cannot adjust rotation if the femoral neck has torsional deformities, making other prosthesis types preferable in such cases to avoid dislocation risks.

Tapered, fluted femoral stems, first proposed by Heinz Wagner in 1987, have shown good spontaneous bone growth in mid-term follow-ups. Factors such as low elastic modulus and mechanical stability influence new bone formation. An in vitro study by Sangiorgio indicated that tapered stems offer better axial stability than cylindrical stems. Only tapered stems achieve stability when proximal bone loss exceeds 105mm. The taper provides axial stability, and flutes enhance rotational stability.

lPaprosky Type III Defects

Type III defects feature severe metaphyseal bone loss with no proximal support and significant diaphyseal deficiencies, with only the isthmus retaining sufficient bone for stability. Type III defects are further classified into IIIA and IIIB based on whether the isthmus integrity exceeds 4 cm (Figure 3 A-B). This subtype distinction is based on Paprosky's clinical observation that cases with less than 4 cm of isthmus integrity have poorer prognoses.

Figure 3 A-B: Paprosky Type IIIB defect shows severe proximal bone loss with no metaphyseal support. Despite diaphyseal deficiencies, the isthmus retains over 4 cm of bone.

For IIIA defects, extensively coated cylindrical stems or modular/monoblock tapered stems are used for revisions. Clinical outcomes of modular or monoblock tapered stems in Type III defects are similar to those in Type II defects (Figure 4 C-D). Using monoblock stems in heavy or narrow canal cases avoids prosthesis fracture at the modular junction. The proximal part of modular stems is relatively thicker, making placement challenging without ETO in narrow canal cases.

Figure 4 C-D: Modular tapered fluted prostheses effectively address these defects.

Tapered stems are often used in cases with femoral canals exceeding 19mm. Several authors report good early and mid-term survival rates for modular and monoblock tapered stems, though long-term results are lacking. However, with increasing patient longevity, the in vivo survival rates of tapered stems are expected to improve. The taper design ensures axial stability, while ridges or flutes on the prosthesis surface provide rotational stability (Figure 5 A-C). Unlike EPC stems, tapered stems do not require strict rotational alignment, allowing adjustments for proximal femoral deformities. Stress distribution promotes excellent bone integration.

Figure 5 A-C: Modular tapered fluted titanium stems are a viable option for Type IIIB defects.

There are concerns about modular femoral prosthesis component fatigue and fracture, with risk factors including patient weight, stem thinness, and ETO. Therefore, monoblock stems are recommended for patients with narrow proximal femurs to avoid prosthesis fracture.