A tailor-made and economical solution for precise interface between hip stem and joint ball.

Artificial hip joints have to be manufactured with high precision, especially in the area where the hip stem and the joint ball connect. The machining solution developed by CERATIZIT is essentially based on a u-axis system and a stepped reamer. These (and other) precision tools ensure that all required tolerances are met while increasing productivity in comparison to conventional production methods

If a hip joint is affecting their quality of life by restricting movement and causing chronic pain, and if conservative treatment methods are no longer helping, many people decide to have an artificial replacement joint implanted. Total joint replacement surgery, including hip replacement, is one of the most commonly performed elective surgeries. 

According to the American Academy of Orthopaedic Surgeons (AAOS), more than 450,000 total hip replacement surgeries are done in the United States every year. This number is expected to reach 635,000 surgeries annually by 2030. Everyone who takes this route is hoping for long-lasting improvements. In order to make their wish a reality, as well as a good surgeon and first-rate care, the highest quality "spare parts" are needed.

Prosthetics like this usually consist of a hip stem with joint ball, a hip socket and an intermediate piece to ensure the movement is as smooth as possible. Particular attention should be paid to the connection between the hip stem and the joint ball. In order for the conical surfaces to fit together perfectly, they need to be produced with the highest precision and surface quality.

The tools used play a crucial role when manufacturing these components. Dirk Martin, our Application Manager Medical explains: "An artificial hip joint consists of difficult-to-machine materials which not only need to be machined within the narrowest tolerances but also as economically as possible. Ultimately, an artificial hip replacement of the highest quality should be available to as many people as possible. We work with great dedication to find suitable tool solutions for these tasks."

A wealth of experience and a wide range of tools are both helpful when searching for a solution

CERATIZIT is a full-range provider in the machining sector that has a wide range of standard and specially-made tools as well as in-depth machining expertise at its disposal. "With our huge product range and the expertise of our application specialists, we are extremely well equipped for tasks like machining the area where the hip stem and joint ball connect," stresses Application Manager Martin. "With our range of tools, we can test all manner of approaches to ultimately find the optimal solution."

In the case of the artificial hip joint, the customer has particularly demanding and varied requirements. For the hip stem, made from high-strength titanium alloy Ti6Al4V, an angle tolerance of just +/-5' must be observed in the conical connection area. Other tolerances are 3 µm for straightness, 8 µm for roundness and 60 µm for the diameter. It is also important that the specified contact ratio for the cone is achieved and a precisely defined groove profile produced.

The joint ball is made from a cobalt-based alloy (Co-Cr-Mo). Its conical hole must have the same shape, angle and dimension tolerances, as well as the specified contact ratio. However, there must be no marks, ridges or grooves made during machining. Dirk Martin mentions another crucial factor: "We need a production solution that is suitable for mass production. This means the machining must be process-secure and require as little monitoring as possible."

Flexible u-axis and special conical reamer ensure success

To produce the conical outside profile, we have opted for pre-machining with a solid carbide conical milling cutter. The subsequent roughing and finishing is performed using a u-axis system. "This is an interchangeable, freely programmable NC axis for machining centers, which can be used to machine contours or for turning." explains Dirk Martin. "Attachment tools and indexable inserts can be used to implement contours in holes and external machining work. This usually means that production times can be reduced considerably, while providing optimal surface quality and higher shape accuracy than usual." This means the desired groove structure can be produced on the stem cone even on a machining center. This has the benefit that all machining processes can be done on a single machine. Using the conventional process, a lathe and a milling machine would be required, which means additional clamping, aligning, time and money.

To make the conical hole in the joint ball, our solution involved the following steps being carried out on a lathe: First, the part is faced to provide a straight chamfer for the subsequent special solid carbide 180° drill with four cutting edges. This is then used to make a hole with a flat bottom. After this an EcoCut Classic drill and turning tool is used to produce the cone with close-contour boring, while a special solid carbide conical reamer ensures the ideal contact pattern and perfect surface quality and tolerance is achieved. The regrinding capability also saves the user further production costs. Dirk Martin comments: "We were also able to produce the internal contour to the required precision with a u-axis. However, using the reamer is the better method for the ultra-smooth surface quality required."

Prospects for the future

All in all, our approach enables the critical contours of the artificial hip joint to be produced process-secure and extremely economically. The contact pattern achieved (contact ratio and contact surface) and the tolerances for both machining processes are well below the specified limits. Dirk Martin is proud of this team achievement: "With this approach, we were able to position ourselves as a full-service provider for our customer, a renowned medical technology company, with the prospect of further orders." The tool specialist is also recommended for similar applications, such as in the area of artificial shoulder joints (also with stem and joint ball), as well as for attachments for artificial knee joints.

Our solution at a glance and specific values

Conical outside profile of the hip stem (produced on a machining center):

  1. Pre-machining with a special solid carbide milling cutter with machined angle
    - Ø 12.0 mm / z4
    - Vc 90 m/min, Fz 0.12 mm, Apmax 16 mm
  2. Further machining with u-axis with special indexable insert optimized with R0.8
    - Rough cut with Ap(1): 0.20 mm and Fz(1): 0.10 mm
    - Finishing cut with Ap(2): 0.03 mm and Fz(2): 0.24 mm

Conical hole in the hip joint (produced on a lathe with a reamer):

  1. Pre-machining using special 180° drill (special solid carbide 180° drill D12 with 4 cutting edges) to create hole with flat bottom
  2. EcoCut used for close-contour boring (EcoCut Classic 3xD with XCNT 040102ER CTPP430)
  3. Precision machining special solid carbide conical reamer for hole with 5°42'30" +/-5'