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Objective:
Image-guided hip joint aspiration plays a key role in the work-up of suspected periprosthetic joint infection. Unfortunately, dry taps are common and occur in about 36% of aspiration attempts. The purpose of this study is to evaluate whether needle position relative to the prosthesis is associated with dry tap rate in fluoroscopy-guided hip joint aspirations.

Materials and Methods:
All fluoroscopy-guided hip joint aspiration procedures from January 2013 to February 2023 in patients with primary total hip arthroplasties were included in the study cohort. Two readers independently reviewed the saved fluoroscopic images and evaluated needle tip position as either medial to, over, or lateral to the prosthesis. In cases scored as lateral to the prosthesis, the reviewers measured the needle tip distance from the lateral femoral head-neck junction and whether the needle curved along the lateral aspect of the prosthesis. Interreader agreement for needle position was assessed using Cohen’s kappa and discrepant cases were reviewed in consensus. Cases were excluded if insufficient imaging was available to determine needle position. Data was also collected regarding needle gauge, aspirate volume, and whether the patient had CT, MRI, or ultrasound of the affected hip within 1 month prior to the aspiration procedure. Cases with < 0.5 mL native aspirate volume were counted as dry taps. Associations between needle tip position, needle curvature, and presence of prior imaging and dry tap rate were assessed by chi-square test. Point-biserial correlation was used to measure the association between lateral needle distance and dry tap rate.

Results:
The study cohort consisted of 214 procedures performed in 205 patients with average age of 64. There was almost perfect agreement between the two readers in characterizing needle position (<em>k</em> = .828). Dry tap rate was 40% in cases lateral to the prosthesis (18/45 cases), 40% in cases medial to the prosthesis (10/25 cases), and 34% in cases over the prosthesis (49/144 cases). Dry tap rate was 22% in patients with prior imaging (13/58 cases). In cases with the needle tip lateral to the prosthesis, average distance from the femoral head-neck junction was 0.9 cm (range 0 - 2.25 cm), and 15 cases demonstrated needle curvature. There was no statistically significant correlation between needle position (<em>p</em> = .694), needle tip distance from lateral head-neck junction (<em>p</em> = .572) and needle curvature (<em>p</em> = .519) and dry tap rate. There was a statistically significant association between presence of preprocedure imaging and decreased dry tap rate (<em>p</em> = .012).

Conclusion:
Needle tip positioning did not affect the dry tap rate in fluoroscopy-guided hip joint aspirations. The presence of preprocedure imaging was associated with a decreased dry tap rate, suggesting that obtaining preprocedure imaging may help minimize dry taps.