For many patients it is easy to think that all hip stems are pretty much the same. Sure some might be longer than others or have a different angle or texture, but if you've seen one hip stem, haven't you kind of seen them all? Simply put, no you haven't.
First launched in 2002, the versatile POLARSTEM◊ implant is a cementless hip stem for total hip replacement. In 2014, it was introduced in the US and continues to be one of Smith & Nephew's most popular stems thanks to its unique shape and texture characteristics.
The shape of every hip stem is determined by several factors including the type of hip replacement being done, the surgical technique being used and the long-term outcomes for the patient. For example, if you were to look closely at the POLARSTEM implant and compare it to other stems, you may notice that the top of the POLARSTEM is wider. This was done to help reduce the possibility of the stem moving downward into the bone after surgery. You may also notice that the bottom of the stem is shorter and features a narrower tip. Again, this was done on purpose to make the stem easier to implant through the smaller incisions used in today's minimally invasive surgical techniques such as the direct anterior approach.
Perhaps even more obvious than the differences in shape between POLARSTEM and other hip stems is the difference in texture. This is because POLARSTEM features an advanced surface texture of titanium plasma and a hydroxyapatite layer.
Before we explain what this means, it may be useful to explain why you have seen the term "cementless" used so often on this page.
Your hip stem can be held in place in one of two ways, each with its own advantages and disadvantages.
Cemented: The first option uses a substance called bone cement to coat the stem before it is placed in position inside your bone. The advantage to this type of "cemented" stem is that it quickly forms a strong bond between the implant and the bone. However, over time this bond can get weaker and may cause the stem to loosen.
Cementless: The second option is to use an implant that has a porous surface texture that, after it is inserted into it position, allows new bone to grow on and in to fill the open spaces of the texture and form a permanent bond between the implant and the bone. Unlike cemented joints which are stronger sooner and may weaken over time, cementless joints are weaker at first but will get stronger over time.
So what does titanium plasma and a hydroxyapatite layer mean?
Every POLARSTEM is machined out of titanium bar stock. The stem starts out smooth and the porous texture is applied later.
This porous texture is made up of two components, a titanium plasma spray and a substance called Hydroxyapatite (HA). The first component, the titanium spray, provides the structural properties of the texture - the small open spaces necessary for new bone to grow into the implant and lock it in place. The second element, HA, is a calcium phosphate material used in a variety of orthopaedic and dental implants as a synthetic bone substitute.
Important safety notes
Hip replacement surgery is intended to relieve hip pain and improve hip function. However, implants may not produce the same feel or function as your original hip. There are potential risks with hip replacement surgery such as loosening, fracture, dislocation, wear and infection that may result in the need for additional surgery. Longevity of implants depends on many factors, such as types of activities and weight. Do not perform high impact activities such as running and jumping unless your surgeon tells you the bone has healed and these activities are acceptable. Early device failure, breakage or loosening may occur if you do not follow your surgeon's limitations on activity level. Early failure can happen if you do not guard your hip joint from overloading due to activity level, failure to control body weight, or accidents such as falls. Talk to your doctor to determine what treatment may be best for you.