The key to the success of total hip arthroplasty

The new materials currently used in hip prostheses, together with advances in planning and surgical technique, have meant that total hip prostheses are no longer considered the last option for patients who suffer from the wear and tear of this joint. This surgery, reserved until a few years ago for extreme cases, has become the treatment of choice due to its high success rate.

The excellent results of this surgery are due in part to the increased functionality and durability of the new hip prostheses that can last up to 30 years without requiring a replacement, but also to the excellence of the surgeon and the rigorous pre-operative plan: "it is important to expand and share knowledge with other experienced colleagues about hip arthroplasty, an area that is constantly evolving and demands continuous training".

Total hip arthroplasty (TCA) is considered the most effective procedure for patients with advanced hip arthrosis. Although the overall rate of complications is low, many of them can be avoided with proper planning and technique. The preoperative planning of a CTA is performed using templates superimposed on the radiological image simulating a reconstruction of the joint that allows to calculate the size and proper positioning of the prosthetic components. In order to plan the correct correction of deformities or to place implants or prostheses, it is necessary to know as accurately as possible the distance between the hips and the x-ray.

For this purpose, the percentage of magnification of the image needs to be established. In the hip joint, due to the distance from the patient's bone to the receiver of the x-ray apparatus, is usually magnified 20% on average, being in obese patients of more than 25% and in thin patients less than 15%.

The King RJ et al. working group of the University Hospitals Coventry (Coventry, England) have developed a two-plane calibration system. This method allows, using a mathematical formula, to calculate the percentage of magnification according to the size variations of these markers. The aim of this work is to evaluate the radiography calibration system in two planes assessing its usefulness when establishing and optimising the correct implant with the planning software, to determine the anthropometric variables of the patient that may influence the calibration as well as to carry out a preoperative and postoperative radiographic analysis establishing associations with the functional results of the intervention. Hypothesis Performing digital preoperative planning of calibrated radiography in two radiographic planes results in superior (research hypothesis Hi), similar (null hypothesis H0) or inferior (alternative hypothesis Ha) than using calibration by applying a fixed magnification to the PACS. Material and Method Descriptive observational clinical study of concordance on a cohort of patients with radiological evaluation of calibration systems in standard image tests, in a hospital setting. In addition, a prospective observational longitudinal clinical study on the patient cohort was conducted with the aim of clinically evaluating the results of the intervention during a minimum follow-up of two years. Consecutive cohort of 97 patients intervened by non-cemented CTA. Preoperative planning was performed on each patient using simple radiographs calibrated in two different ways; the traditional way (control group) with calibrated radiographs applying a fixed magnification at 20%, and using a new double marker calibration system (experimental group)

Results The concordance of digital preoperative planning of uncemented CTA, when using a double marker calibration system, is 20% more accurate than using fixed magnification for radiograph calibration. The double marker calibration system is useful in 97% of patients and allows measurements with a margin of error up to 4 times less than using fixed magnification. When fixed magnification is used to calibrate preoperative radiography we find anthropometric variables of the patient, such as the abdominal fold, which influence the accuracy of digital planning. Intervened patients improved 7 points on the EVA pain scale and on the HHS scale went from 41.6 preoperative points to 93.9 points at the end of follow-up. Short-term (2-4 years) TCA clinical outcomes are independent of radiographic outcomes. Satisfactory radiographic results are obtained after TCA as planned in more than 70% of patients. Discussion Preoperative planning is fundamental and necessary prior to any surgical intervention. We find numerous legal, educational and above all surgical reasons that force us to perform it properly. Due to the use of digital x-rays in the vast majority of TOC centres, it is necessary to implement digital technical aids for their correct performance. This includes the use of calibration markers, digital templates, specific software or digital planning systems.