Fitmore®
Inspired by Individual Anatomy
Optimal reconstruction of offset is a crucial part of today’s Total Hip Arthroplasty. Conventional stems often fail to match individual anatomy. In most cases offset is linked to stem sizing and as a result, one’s ability to restore natural joint kinematics can be limited.
The Fitmore Hip Stem shape is the result of close observations of individual anatomy in a large patient population in Europe and the USA. In order to provide optimal reconstruction options one important step became obvious during development: offset must be independent from stem size. As a consequence, three different medial stem curvatures
were designed.
Given today’s younger and more active patients consideration was also devoted to preservation of muscle insertions and bone in the region of the greater trochanter and compatibility with less-invasive surgical techniques. The curved stem shape meets both requirements and eases implantation.
Primary stability is achieved by the press-fit fixation and by a triple-tapered design that allows even load distribution. Finally, a clinically proven* Porolock® Ti-VPS Coating in the proximal part was chosen to enhance bone ongrowth.
* i.e. Ingrowth surfaces: Plasma spray coating to titanium alloy hip replacements; RB Bourne, CH Rorabeck, BC Burkart, PG Kirk, Clinical orthopaedics and related research, Vol 298, 1994; p37–46
Design Rationale
Design Features at a Glance
1. 3 different Stem Families with 4 Offset Options
• Small stems with large offsets and vice versa
• Reduces the risk of inadequate reconstruction of length/offset
• Medial curvature that closely follows the natural cortex in order to enhance rotational stability
2. Curved Stem Design
• Preservation of bone in the region of the greater trochanter and of gluteal muscle insertions
• MIS enabled
3. Shortened and Flattened Stem
• Cancellous bone preservation
• Possibility to correct for a pathologic ante- or retroversion
4. Porolock TiVPS and Protasul® 64
• Clinically proven pure titanium coating supporting primary stability and secondary biological fixation
5. Optimized Neck Geometry
• V-shaped neck or slim neck/short cone geometry to optimize range of motion across all families
6. Proximal Fixation
• m/l: medial curvature and lateral cortex
• a/p: 3-point fixation
7. Triple Taper and Trapezoidal Cross Section
• Supports axial and rotational stability
• Enhances even distribution of load within the metaphyseal region
Research Supporting the Design
When using conventional stems, surgeons are often forced to compromise offset in order to accommodate a narrow medullary canal. Offset should not be dictated by stem size. This was the starting point for the designers of the Fitmore Hip Stem. In the belief that this stem should derive rotational stability from contact in the calcar region, fit in this region was also a priority.
A study of 497 x-rays conducted in Switzerland1 confirmed that the final design of the Fitmore Hip Stem covered nearly all measured patient offsets:
In addition, preoperative planning was performed for 100 out of the 497 patients x-rays:
• 46% of patients were male and 54% were female, with an age range between 24 and 86 years
• The Fitmore Hip Stem fit all cases • Families B and B Ext covered 69% of patient anatomies
• Families A, B, and B Ext covered 93% of patient anatomies
• Sizes 4 to 9 covered 75% of patient Anatomies
In order to confirm the 3-D fit of the stem design 139 cases were virtually implanted in American cadaveric femurs:
• 47 females and 92 males
• The Fitmore Hip Stem fit in all investigated femurs
• Families B and B Ext covered 70% of all femurs
• Families A, B and B Ext covered 99% of all femurs
• Sizes 5 to 10 covered 79% of all femurs
1 Unpublished, Zimmer internal data
Reconstruction of the Individual Anatomy
With conventional uncemented stems a surgeon may be forced to use a relatively small stem to fit a narrow canal and thus compromise neck length and total offset. Conversely, a relatively capacious canal may require a large stem with too much leg length and offset.
Inability to recreate offset can lead to bone–bone impingement. Resolving such a solution can require excessive resection of bone and the use of skirted heads.
The Fitmore Hip Stem offers the distinct advantage that the growth of the offset is independent from the growth of the stem body: a step closer to respecting the individual anatomy. As a result the portfolio consists of:
• Stems with larger offset that accommodate thinner medullary canals (small stem body)
• Stems with smaller offset that accommodate larger femoral canals (large stem body)
Moreover, as the stem size grows in the lateral direction within the same family, a change of offset can be done without affecting leg length.
The Fitmore Hip Stem Offers a Combination of Innovative Design Elements:
• Each stem family has a different offset and medial contour which allows for a more accurate reconstruction of the patient’s femoral anatomy.
• The stem is available in four offset versions with three different stem bodies.
• A large offset range, from 31 to 59 mm, allows for a more precise reconstruction or optimization of patient’s offset.
Offset Range
Family A: 31 mm - 39 mm
Family B: 37 mm - 45 mm
Family B Ext: 44 mm - 52 mm
Family C: 51 mm - 59 mm
The offset range is even wider considering the different head sizes.
Bone-conserving and Muscle-sparing Design
The short curved stem design of Fitmore Prosthesis follows the anatomical pathway and therefore conserves bone and protects muscles during preparation of the implant bed. Bone conservation is an important part of total hip arthroplasty. By removing as little bone as possible during a primary operation, more options for any potential future revision are left.
The Fitmore Hip Stem’s short length and curved design preserve the patient’s bone in the trochanteric region. The Fitmore Rasps and Implants allow for a curved trajectory with insertion following the calcar arch. Therefore it is not necessary to interfere with the trochanteric bone and the muscle insertions.
A Stem Adapted to MIS and not MIS Adapted to a Stem
As the Fitmore Hip Stem broaches follow the curvature of the femoral neck, it can be a less-invasive implant and therefore well suited for minimally invasive surgeries.
The stem is not linked to one approach but can be implanted by any of the less-invasive routes (with the exception of the MIS 2-incision technique).
Specially adapted rasp handles help the surgeon to perform his/her preferred approach.