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, 475 (2), 484-494

Periprosthetic Occult Fractures of the Acetabulum Occur Frequently During Primary THA

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Periprosthetic Occult Fractures of the Acetabulum Occur Frequently During Primary THA

Kazuhiro Hasegawa et al. Clin Orthop Relat Res.

Abstract

Background: Periprosthetic fractures of the acetabulum occurring during primary THA are rare. Periprosthetic occult fractures are defined as those not identified by the surgeon during the procedure which might be missed on a routine postoperative radiograph. However, it is unclear how frequently these fractures occur and whether their presence affects functional recovery.

Questions/purposes: In this study, using routine CT scans that were obtained as part of another primary hip arthroplasty study protocol, we retrospectively assessed (1) the prevalence of occult fractures of the acetabulum occurring during primary THA, (2) the location of occult fractures of the acetabulum during THA, and (3) risk factors contributing to such occult fractures.

Methods: Between 2004 and 2013, our institute performed 585 primary THAs (cementless or hybrid) in 494 patients with DICOM pre- and postoperative CT; during the period in question, all patients undergoing THA underwent CT before and after surgery. Preoperative CT images were taken as part of a CT-based three-dimensional templating software and navigation system. Postoperative CT images were taken an average of 1 week after surgery as part of a different protocol to evaluate cup position, restoration of leg length and offset, volume of postoperative hematoma to assess anticoagulation effects after THA, and fractures that were not found on routine postoperative radiographs (which we defined as occult fractures). Patients with a history of prior pelvic osteotomy, trauma, and infection were excluded (88 patients/99 hips); 406 patients (102 males and 304 females; 486 hips) form the basis of this report. The mean age of the patients was 60 ± 11 years, with a mean BMI of 23 ± 4 kg/m2. The mean followup of the patients with periprosthetic fracture of the acetabulum was 58 ± 28 months (range, 12-131 months). Potential risk factors for occult acetabular fracture including age, sex, BMI, preoperative diagnosis, additional dome screw fixation, composition and size of each cup, and acetabular design were examined in multivariate analysis. Acetabular component designs were categorized as true hemispheric, peripheral self-locking, and elliptical; during the period in question the indications for each cup design were based on the brand of stem used. Comparison between preoperative and postoperative CT images was done to detect the fractures. Patients with fractures identified during surgery were treated with additional dome screw fixation and a 3-week period of nonweightbearing. Patients with occult fractures in this series did not receive additional treatment as we had confirmed secure fixation of the cup during surgery.

Results: Occult fractures occurred in 41 hips (8.4%); periprosthetic fractures of the acetabulum were seen during surgery in an additional two hips (0.4%). The superolateral wall was the most frequent location for occult fractures of the acetabulum. After controlling for relevant confounding variables, only the use of peripheral self-locking cups was associated with an increased risk of occult fracture (odds ratio [OR], 2.6 compared with hemispheric cups; 95% CI, 1.2-5.6; p < 0.05). All patients with occult fractures showed bone ingrowth fixation at the final followup, without any additional surgical intervention.

Conclusions: Periprosthetic occult fractures of the acetabulum may occur relatively frequently during press-fit impaction. We observed a higher rate of fractures associated with the use of peripheral self-locking components. Occult acetabular fractures not detected on routine postoperative plain films may be ignored if secure fixation of the cup has been confirmed during the operation.

Level of evidence: Level III, therapeutic study.

Figures

Fig. 1
Fig. 1
The diagram shows acetabular component designs of hemispheric, peripheral self-locking, and elliptical cups. Elliptical cups have a peripheral flare 2 mm larger than a cup diameter and peripheral self-locking cups have a rim that is 1.8 mm larger than the cup diameter.
Fig. 2A–D
Fig. 2A–D
We defined occult fractures as those that could be confirmed on the postoperative CT images but were not seen intraoperatively or found on postoperative radiographs. We reconstituted pre- and postoperative CT images (approximately 1 week after surgery) to (A) coronal, (B) sagittal, and (C) axial images and diagnosed an occult fracture when we were able to confirm a fracture (arrow) line in two or more planes on the images. (D) We were able to confirm a fracture (arrow) line in the three-dimensional image.
Fig. 3A–T
Fig. 3A–T
(A) The diagram shows an occult fracture in the medial wall. CT reconstruction shows the occult fracture (arrow) line on the (B) axial, (C) sagittal, and (D) coronal images. (E) This diagram shows an occult fracture on the posterior wall. CT reconstruction shows the occult fracture (arrow) line on (F) axial, (G) sagittal, and (H) coronal images. (I) An occult fracture on the superolateral wall is shown. CT reconstruction shows the occult fracture (arrow) line on (J) axial, (K) sagittal, and (L) coronal images. (M) An occult fracture on the anterior wall is shown. CT reconstruction shows the occult fracture (arrow) line on the (N) axial, (O) sagittal, and (P) coronal images. (Q) An occult fracture at the area classified as “other location” is shown. The CT reconstruction shows the occult fracture (arrow) line on (R) axial, (S) sagittal, and (T) coronal image.
Fig. 3A–T
Fig. 3A–T
(A) The diagram shows an occult fracture in the medial wall. CT reconstruction shows the occult fracture (arrow) line on the (B) axial, (C) sagittal, and (D) coronal images. (E) This diagram shows an occult fracture on the posterior wall. CT reconstruction shows the occult fracture (arrow) line on (F) axial, (G) sagittal, and (H) coronal images. (I) An occult fracture on the superolateral wall is shown. CT reconstruction shows the occult fracture (arrow) line on (J) axial, (K) sagittal, and (L) coronal images. (M) An occult fracture on the anterior wall is shown. CT reconstruction shows the occult fracture (arrow) line on the (N) axial, (O) sagittal, and (P) coronal images. (Q) An occult fracture at the area classified as “other location” is shown. The CT reconstruction shows the occult fracture (arrow) line on (R) axial, (S) sagittal, and (T) coronal image.
Fig. 3A–T
Fig. 3A–T
(A) The diagram shows an occult fracture in the medial wall. CT reconstruction shows the occult fracture (arrow) line on the (B) axial, (C) sagittal, and (D) coronal images. (E) This diagram shows an occult fracture on the posterior wall. CT reconstruction shows the occult fracture (arrow) line on (F) axial, (G) sagittal, and (H) coronal images. (I) An occult fracture on the superolateral wall is shown. CT reconstruction shows the occult fracture (arrow) line on (J) axial, (K) sagittal, and (L) coronal images. (M) An occult fracture on the anterior wall is shown. CT reconstruction shows the occult fracture (arrow) line on the (N) axial, (O) sagittal, and (P) coronal images. (Q) An occult fracture at the area classified as “other location” is shown. The CT reconstruction shows the occult fracture (arrow) line on (R) axial, (S) sagittal, and (T) coronal image.
Fig. 4
Fig. 4
The locations of occult fractures are classified in five areas—the (1) medial wall, (2) posterior wall, (3) superolateral wall, (4) anterior wall, and (5) other locations.
Fig. 5A–H
Fig. 5A–H
CT reconstruction shows the scans for a 64-year-old man with osteonecrosis of the femoral head. A peripheral self-locking cup with 1.8-mm underreaming was used for the rim diameter. A periprosthetic occult fracture (arrow) of the acetabulum was seen on postoperative CT images. CT reconstruction shows the occult fracture (arrow) line on (A) coronal, (B) sagittal, (C) axial and (D) three-dimensional images. The occult fractures that had achieved bone union (arrow) 3 months later is seen on these (E) coronal, (F) sagittal, (G) axial, and (H) three-dimensional CT reconstruction images. The implant had bone ingrowth with no malalignment. The patient had no additional treatment.

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