## 2 Materials and Methods  
This prospective clinical study was approved by the local IRB (4565) and registered in the German Clinical Trail Register (Trail-ID: DRKS00010421). The primary aim of the project was to compare the gait pattern between patients who received a short-stem total hip replacement (METHA, Fa. Aesculap, Tuttlingen, Germany) and cementless cup (PLASMAFIT, Fa. Aesculap, Tuttlingen, Germany) through a standard lateral approach and a minimal-invasive antero-lateral approach. Power analysis for this yielded a size of 30 patients in each group. The current study question regarding the impact of a FAI type cam represents a secondary aim. Thus, all results only represent assumptions. All patients provided an informed consent. After written informed consent, the patients were consecutively included with respect to the following inclusion and exclusion criteria.  
* end-stage, therapy-resistant osteoarthritis of the hip in patients with ages between 18 and 75 years on the day of surgery;
* no deficiency in the motoric and neurologic function of the lower extremity, documented by the preoperative clinical examination.
* Exclusion criteria:
* cardiovascular disease as contraindication for physical exercise, e.g., cardiac insufficiency (NYHA class IV);
* intake of drugs affecting the patient's balance;
* changes in balance and sensitivity affecting the lower extremity due to diseases such as Meniere's disease, complete loss of eyesight, polyneuropathy, multiple sclerosis;
* previous bony surgery on the affected hip;
* arthrodesis or replacement of the knee or ankle joint.  
All patients sustained an end-stage hip OA and were planned for THA. Based on the axial radiograph of the hip, the alpha angle described by [PERSON] was used to determine the incidence of a CAM impingement in the patient population (alpha angle > 65\({}^{\circ}\)) [16]. As patients with primary OA may build osteophytes around the femoral neck junction, we decided to enhance the usually used cut-off value for a FAI type CAM from 55\({}^{\circ}\) to 65\({}^{\circ}\) to achieve a clearer selection between patients with osteophytes and a bony bump. Furthermore, the offset and the caput-collum-diaphyseal (CCD) angle was measured on the native hip joint of the preoperative anterior-posterior (a.p.) pelvic planning radiograph, standardized in the supine position, to define the hip morphology [17,18]. The measurements were repeated after hip replacement on the control a.p. pelvic at the 12-months follow-up examination according to our clinical routine. Direct postoperative radiographs were not used for evaluation due to possible inaccuracies caused by postoperative pain resulting in limitations of positioning. All radiographs were analyzed with Carestream(c) Carestream Solutions, Vue Solutions).  
The clinical observations were published [19; 20]. However, to compare the preoperative findings with the 12-month follow-up results between patients with or without an FAI type CAM, the Harris Hip Score (HHS) was added.  
From the original total study group of 60 patients, five patients (8.3%) were subsequently excluded due to more than two missing follow-up examinations. Two further patients missed two (3.3%) and 19 one (31.6%) follow-up examinations. As a result, 34 patients (56.8%) attended all follow-up points. In the case of participation, all data were attained in total. In accordance with our biometric procedure, the missing values for the 21 patients with one to two follow-ups less could be filled with the results of the previous values of the earlier one to two examination time points.