First experiences with the Fitmore ® hip stem – Early results of the 16-month monitoring

Background/Aim. Fitmore® hip stem belongs to the group of short stem prostheses with the metaphysar stabilization, with its shape and form that protects the bone mass in the greater trochanter region and the distal part of the femur. The aim of this paper was to present the early postoperative results in patients with implanted Fitmore® hip stem and point out some of the advantages. Methods. A series of 10 patients with implanted Fitmore® hip stem, was included in this study. The average age of the patients was 54.5 (48–65) years. There were 5 women and 5 men. The total monitoring time was 16 months. To rate the condition of the hip joint we used The Western Ontario and Mc Master Universities Arthritis Index (WOMAC) score. We also monitored the degree of hip pain, hip flexion, heterotopic ossification and indentation in the stem of the prosthesis. Results. After 12 months of monitoring 9 (90%) of the patients had no pain in the thigh region, and only 1 (10%) experienced mild pain. The hip flexion rose from the average 89° to postoperative 114°. WOMAC score rose as well, from 49 to 94 average points. Indentation in the stem was registered 3 months after the operation in 2 (20%) of the patients – in one of the patients the indentation was 3 mm and in the other patient 5 mm. After the 16-month monitoring, the results were excellent. The monitoring period was short though it should be continued and the results should be presented after 5 and then after 10 years. Conclusion. Early results of the implantation Fitmore stem showed good bone ingrowth with excellent functional result.


Introduction
Osteoarthrosis is a very common chronic disease. It is detected in 60% of the population aged over 65, and hip osteoarthrosis is detected in 5% of the population aged over 55 1 . Etiology of coxarthrosis is complex and depends on multiple factors, all of these factors, individually or combined, can cause a degenerative hip disease.
Replacement of the natural hip joint with an artificial one restores the function of the diseased joint and establishes a painless and satisfactory locomotion. There are a number of cemented and uncemented prosthetic models used in Correspondence to: Marko Mladenović, Clinic for Orthopaedic and Traumatology, Clinical Centar Niš, Dragiše Cvetkovića 21/1, 18 000 Niš, Serbia. E-mail: mladenovicdmarko@gmail.com everyday practice. One of the models is Fitmore hip stem by Zimmer. Fig. 1 -Implanted Fitmore ® hip stem (larger calcar radius, A family).
This particular system has a short uncemented stem which with its shape and curve restores the anatomy of the proximal part of the femur and allows adjustment of the prosthetic offset, ie it offers the possibility of adjusting the distance between the acetabulum and body of the stem. The transection of the stem has a trapezoid form, which allows the primary rotational stability. It belongs to the group of short stem prostheses with metaphysar stabilization and therefore it spares the bone mass in the greater trochanter region and the distal part of the femur 2 . With its curve, the stem of this prosthesis protects the lateral cortex of the femur, and directs its contact and the transmission of the mechanical forces and loads in the hip, towards the small trochanter. The greatest part of body weight is transferred through the medial part of the prosthesis towards the small trochanter, to the medial part of the resected femoral neck which has the shape of the letter U -this is calcar femoris. The surface of the cortical bone part of the resected femoral neck in the region of the small trochanter, which is used for the transfer of weight, is about 1.29 cm 2, 3 .
The short and curved prosthetic stem spares the distal femur part, this maladjustment of the proximal and distal part is avoided, which is one of the reasons for the loosening of the long prosthetic stem. Additionally, it spares the femur canal for the revision stem; it reduces the intraoperative bleeding and spares the soft tissue due to the reduced surgical exposition 4 . Fitmore ® prosthesis has a narrower indication area, ie it is used in younger patients, vital and good physical shape with a small degree of osteoporosis.
The aim of this study was to present the early postoperative results in patients with the implanted Fitmor ® hip stem and point out some of its advantages.

Methods
In the period from December 25, 2013 to February 15, 2014 in the Clinic for Orthopedic Surgery the Niš, Serbia, ten hip endoprostheses of the Fitmore stem type were implanted.
Total hip arthroplasty was performed in 5 female and 5 male patients. The average age was 54.5 years (57.4 in male and 50.4 in female patients). The average weight was 81 kg in male and 69 kg in female patients.
The patients were monitored after 3, 6, 12 and 16 months following the operation.
We used the intermediate calcar radius stem in 8 patients, and in the remaining 2 a larger calcar radius stem A family (Figure 1).
To rate the state of the hip joint, prior to and after the operation, we used WOMAC score 5 .
It is critical with the use of the Fitmore ® stem to use radiological templating to determine the appropriate stem family preoperatively, as medial metaphyseal/diaphyseal contact cannot be visualized intraoperatively. Templating is needed to assess the center of femoral head, leg length, offset, level of femoral neck osteotomy and stem size. The neck osteotomy must be at an angle of 50° of the long axis of the shaft and also preservation of 5-10 mm of the lateral neck cortex.
Subsidence was defined as vertical stem movement of more than 5 mm according to Callaghan et al. 6 .
Heterotopic ossification was classified according to Brooker et al. 7 .

Results
Three months after the operation, in 5 (50%) patients thigh pain disappeared, and in 9 (90%) after 12 months. Only one (10%) patient had mild pain 12 months after the operation.
Radiological results after 16 months of monitoring were in 2 (20%) of the patients hipertrophy of the femoral cortex at the height of the prosthetic stem was registered, but without clinical symptomatology. In 1 (10%) of the patient heterotrophic ossification gradus 1 according to Brooker et al. 7 was registered, in 2 (20%) of the patients there was an indentation in the stem up to 5 mm, registered 3 months after the operation.

Discussion
Total hip arthroplasty is one of the most successful orthopedic procedures. The use of short stems is growing. With short stems good and permanent fixation is achieved and clinical results are good.
Joint Implant Surgery and Research Foundation (USA) has formed a classification system for the short stem uncemented prosthesis: head stabilized, neck stabilized, metaphyseal stabilized and conventional (metaphyseal/diaphyseal) stabilized 8 . Fitmore ® hip stem by Zimmer that we used in our patients belongs to the group of metaphyseal stabilized, and its characteristics are the following: it spares the bone mass in the area of greater trochanter and diaphysis of the femur, crossection is trapezoid and provides excellent stability, it has different curves in order to renovate the hip joint anatomy and achieve a good offset of the femoral neck 4, 8, 9 . Radiological templating is mandatory to determine the position of the prosthesis, its size, offset center of the rotation and leg length 10 .
A long prosthetic stem can be implanted in the varus position. Berend et al. 11 , Khalily and Lester 12 mention the varus of prosthetic stem from 4° to 8° as well as the fact that Fitmore ® stem has a small potential to take the varus position. Additionally, sometimes a fracture of the long stem can occur.
Loosening of the stem did not occur in our series, perhaps due to the short monitoring period, although other authors do not list it either. This phenomenon occurs in prostheses with long stem due to different factors 13 .
The stem of the Fitmore ® prosthesis can cause remodeling of periprosthetic bone structure. Pepke et al. 14 has found that both Fitmore ® and long stem prostheses have proximal stability. Fitmore ® stem also has rotational stability, so its rigidity is higher, which is the reason why the remodeling process of the bone is more prominent in this type of prostheses. Guske 2 as well, in his work reports cortical hypertrophy in 29% of the patients in the series of 100 patients. In our series, this phenomenon was present in 2 (20%) of the patients.
Indentation in the Fitmore ® stem up to 5 mm was present three months after the operation. We had 2 (20%) patients -in one the indentation in the stem was 3 mm, and in the other 5 mm. In the series of 100 patients, Guske 2 reported indentation in 34% of the patients.

Conclusion
The early results of Fitmore ® stem implantation showed good bone ingrowth with excellent functional result in our patients. The number of published series of patients with implanted Fitmore ® hip stem is small, so as in our study of the early results of the implantation of Fitmore ® stem in 10 patients. Further monitoring of the patients and reporting the results 5 and 10 years after the operation are needed. Monitoring in our group of patients was 16 months.