Femoral Stem Dislodgement During Bipolar Hemiarthroplasty Dislocation

Case Report

An 82-year-old woman sustained a closed unstable femoral neck-fracture (Garden IV) as a result of a fall in which she landed directly on her left hip (Figure ). Investigation of her pre-injury status revealed a functionally independent patient in bathing, dressing, using the lavatory, feeding, and transferring with no walking aid. Medical evaluation detected congestive heart disease, chronic obstructive lung disease, hypertension, and slightly deteriorated mental status.

Figure 1:. Preoperative Radiograph of the Left Hip Revealed Unstable Femoral-Neck Fracture (A). Immediate Postoperative AP (B) and Lateral (C) Radiographs Showed Good Alignment and Positioning of the Implants.

Four days after admission, the patient underwent left hip bipolar hemiarthroplasty through a posterior approach with implantation of a cemented polished collarless tapered stem and a bipolar socket designed with an external poly-ethylene locking ring. Immediate postoperative radiographs revealed satisfactory positioning of both implants (Figures , !).

The patient was started on an accelerated rehabilitation program developed and validated at our institution ¹¹ and had an uneventful recovery, walking with the aid of a walker 1 day postoperatively. On postoperative day 3, she was discharged with a well-functioning prosthesis, satisfactory pain relief, and stable medical conditions. During the follow-up period, she was evaluated on postoperative day 8 for suture removal and functional status. She presented with a walker gait, slight pain at the surgical site, and well-functioning prosthesis.

Twenty-nine days postoperatively she was admitted with a painful and shortened leg after a fall from an 8-step staircase. Radiographs of the left hip showed a bipolar-socket dislocation with no disassembly of the modular components, and associated with femoral stem dislodgement out of the cement mantle at the cement/stem interface (Figure ).

Figure 2:. Left Bipolar-Socket Dislocation Associated with Femoral Stem Dislodgement Out of the Cement Mantle (A). AP Radiograph of the Left Hip After Open Reduction. The Femoral Stem Was Driven Back to the Original Cement-Mantle Followed by Reduction of Bipolar Component Dislocation and Placing Bone Cement over the Shoulder of the Femoral Stem (B).

The patient underwent open reduction. Intraoperative findings included a dislocated outer shell with undamaged polyethylene external locking ring mechanism adequately assembled and placed securely on the metal femoral head. The proximal one-third of the femoral stem protruded out of the femoral canal and the cement mantle appeared undamaged. The femoral implant was driven back to the original cement mantle, and impacted with a heavy hammer with the aid of the original stem inserter. No additional bone cement was inserted in the remnant cement mantle. Following reduction of the bipolar component dislocation, a small amount of bone cement was placed over the shoulder of the femoral stem to prevent a new dislodgement episode (Figure ). The stability of the reconstruction was evaluated through limb traction and movements in all directions of the prosthetic hip.

One day postoperatively, the patient was able to walk with the aid of a walker. On postoperative day 4, however, her clinical and mental status progressively deteriorated due to cardiac arrhythmia, and subsequent stroke. She eventually died 15 days postoperatively.

Discussion

Total hip arthroplasty is considered to be the most adequate procedure for the treatment of displaced femoral neck fractures in elderly active patients. ² However, bipolar hemiarthroplasty remains the most frequent procedure performed for less active patients with cognitive dysfunction, mainly due to the lower prevalence of dislocation, which approaches rates 7 times less than that of dislocation after THA. ¹ The clinical failure of bipolar hemiarthroplasty is associated with several implant-related complications, which include acetabular erosion, dislocation, disassembly of the modular components, polyethylene debris-induced osteolysis, metallosis from outer cup impingement and loosening of the femoral component. ^3–5,12

Our patient developed an unreported complication following bipolar hemiarthroplasty manifested by dislodgement of the femoral implant out of the cement mantle during traumatic dislocation. A literature search, conducted to identify reports of femoral stem dislodgment or displacement from the cement mantle following total or partial prosthetic replacement of the hip, revealed this complication to be associated only with attempted closed manipulation of THA dislocation ^7–10 or, less frequently, during the acute episode of THA dislocation. ⁶ In such circumstances the pull-out forces transmitted to the stem may exceed the strength in axial direction at the level of the stem/cement interface.

The fact that the rare cases of femoral stem dislodgement reported in the literature have been invariably associated with cemented polished tapered stems suggests this complication to be strongly related to implant design features. Once the axial stability of the femoral component depends on the strength of the stem/cement interface, the surface roughness of the implant and the resulting coefficient of friction will ultimately determine its resistance to pull-out forces. ¹³ A rough surface creates mechanical interlocking between the implant and the apposed bone cement thus increasing the forces required to disengage the stem out of the cement mantle.

The design conception for decreasing friction between the stem and the cement mantle constitutes the basis for the use of polished tapered implant, which permits the distal migration of the tapered stem in the cement mantle, so creating a favorable mechanical environment for load transmission. ¹⁴ Nevertheless, in this circumstance, the bone cement acts more as a grout than an adhesive and consequently produces a very weak bond and very low friction between the cement and the stem which, in turn, makes the polished tapered stem susceptible to easy disengagement when submitted to pull-out forces. ^15,16 Such a mechanical behavior may also explain the occurrence of stem disengagement in association with undisturbed cement/bone interface, adequate cement mantle and contemporary cement techniques as reported in the literature. ^7–10

In our patient the injury was associated with significant energy, which caused a dislocation of the metallic outer shell. The undamaged external ring transmitted traction forces to the femoral stem through the taper-lock mechanism of the modular head, which remained intact. The lower resistance to pull-out forces of the polished tapered stem at the cement interface, in relation to the metallic head-stem modular taper and the locking ring of the bipolar hemiarthroplasty, was responsible for the femoral stem dislodgement in this patient. According to laboratory studies, the force required to disengage the femoral polished tapered stem out of the cement mantle averages 2060 N and so, may be lower than the forces generated by a posterior hip dislocation estimated by finite element analysis, which range from 1200 to 6000 N. ^16,17 In fact, we ¹⁵ and other authors ^16,18 reported, in clinical and experimental studies, the easy removal of tapered polished stems out of the cement mantle when submitted to pull-out forces.

The decision to drive the stem back to the original cement mantle and to reduce the original bipolar implant was based on the integrity of the stem surface, the cement-bone interface and the undamaged polyethylene external locking ring mechanism, which was adequately assembled and securely fixed on the metal femoral head.

The reinsertion of collarless polished tapered stems into the original cement mantle is not a new procedure. Nabors et al ¹⁸ reported the removal and reinsertion of cemented femoral component into the original cement mantle, during isolated acetabular revision, with excellent results in 42 hips, with no measurable detrimental effects during the first 2 to 10 years since revision. In all but 8 patients the stems were tapped back into the cement mantle with no additional bone cement in the remaining cement cavity.

More recently we conducted an experimental study in anatomical models of hip arthroplasty with 3 different designs of tapered polished collarless stems to evaluate the mechanical safety of reinserting the femoral stems with no additional bone cement inside the original mantle. ¹⁵ We found that reinsertion of same design and size polished tapered stems may not alter the pattern of load transmission and stability at the interfaces between stem/cement, and to the outer surface of bone. This procedure may be used alternatively to the cement-in-cement technique, once mechanical conditions at the interfaces are restored with no need for additional cement during reinsertion if the cement envelope is preserved. The same mechanical principle that maintains the stability of the stem during subsidence for “force-closed” fixation may also keep the stability of the interface following reinsertion. ¹³ For these reasons this procedure may not be applicable to designs with texturing or pre-coating, nor to cylindrical-collared designs, because in such conditions of “shape-closed“ fixation the mechanics of stem/cement interface may not be restored.

These considerations may also explain the reason we could not find any reference to femoral stem disengagement or dislodgement of cementless stems out of the femoral canal. The high friction of the stem/bone interface of such implants, which relies on press-fit or scratch-fit fixation for immediate stability, increases the axial resistance of the implant to pull-out forces. The design conception, for initial stability and biological fixation, also incorporates rough stem surfaces, which enhance the interface resistance to axial forces. However, severe pull-out forces may theoretically disengage undersized loose uncemented stems.

Covering the shoulder of the femoral stem with cement produces a statistically significant increment of the pull-out forces required to dislodge the implant, which means a greater stability in the axial direction. The higher fracture toughness of the bone cement in relation to the fracture toughness of the stem cement interface requires a higher peak force for stem displacement. Although femoral stem dislodgement may be prevented by placing bone cement over the shoulder of the prosthesis, this is not yet a common practice, and should be considered whenever a cemented polished tapered stem is implanted. ¹⁶

In the evaluation of the root causes for postoperative death in this patient, the 4-days delay for fracture treatment played a role in the increased risk for mortality following the index procedure. Several articles and guidelines indicate that surgical delay of hip fracture surgery in elderly patients has a major negative impact on in-hospital and 30-day mortality. ^19,20 However, for this particular patient, recovery was uneventful, and she was active and walking with the aid of a cane by the 29th postoperative day.

Despite the fact that bipolar dislocation is a rare complication, approximately 60% of these episodes require open reduction. ¹ Open reduction is also invariably required for dissociation of the locking ring and modular head disassembly, which may occur during the traumatic event or as a consequence of limb traction during reduction maneuvers. A second operation is usually risky for the elderly patient who sustains a femoral neck fracture. We hereby describe one more reason for compulsory open reduction, which may endanger the patient’s life. The occurrence of femoral stem dislodgement during bipolar hemiarthroplasty dislocation should be considered as a possible complication of such a procedure and may be prevented by routinely placing bone cement over the shoulder of the femoral polished tapered implant.