The cementless total hip replacement (THR) has established itself as one of the most common options for treating advanced hip osteoarthritis and other degenerative hip diseases. Its main advantage is that the prosthesis shaft is not fixed with cement, but instead grows directly into the bone. This ingrowth occurs in two phases: primary stability and secondary stability. A successful ingrowth and the associated long-term durability of the prosthesis depend on several factors, which we will explain in detail in this article.

Primary stability after total hip replacement

Primary stability describes the initial mechanical fixation of the cement-free prosthesis shaft immediately after surgery. It is achieved through the exact fit of the shaft in the femur and is crucial for the initial anchoring of the prosthesis in the bone.

Factors influencing primary stability:

• Anatomical fit: Modern hip prostheses, especially short-stem prostheses, are designed to optimally adapt to the individual anatomy of the femur. A precise fit is crucial to prevent movements of the prosthesis shaft in the first postoperative weeks.
• Surface coating: The rough, porous surface of the prosthesis stem promotes bone ingrowth. Titanium and other biocompatible materials are particularly suitable for this, as they promote high osseointegration.
• Minimally invasive surgical techniques: These techniques enable less damage to the surrounding soft tissues and muscles, which supports the stability of the prosthesis in the early phase.

Secondary stability after total hip replacement

Secondary stability develops over the weeks and months following surgery as the bone grows into the surface of the prosthesis, achieving a permanent, biological anchorage. This phase is crucial for the long-term durability of the prosthesis.

Factors influencing secondary stability:

• Bone healing: The healing process of the bone is individual and can be influenced by various factors, such as age, bone density, and the patient's overall health. Secondary stability occurs as soon as the bone is stably integrated with the prosthesis.
• Surface structure of the prosthesis: The microroughness and special coatings, such as calcium phosphate or hydroxylapatite, promote bone growth and support osseointegration.
• Correct postoperative loading: A controlled, moderate loading of the operated leg protects the ingrowth process of the bone into the prosthesis. However, excessive loading in the early phase can impair ingrowth.

Risks associated with disturbed ingrowth

A non-successful ingrowth of the cementless prosthesis can have serious consequences, such as loosening, instability, or the need for revision. The most common risk factors include:

• Early heavy loading: While it is important to load the operated leg moderately, excessive loading, especially in the first few weeks, can lead to micro-movements between the prosthesis and bone, which interfere with ingrowth. This increases the risk of early loosening of the prosthesis.
• Poor bone quality: Patients with osteoporosis or other bone diseases have an increased risk of disturbed ingrowth, as the bone may not be sufficiently stable to securely anchor the prosthesis.
• Infections: An infection in the surgical area can disrupt the healing process and impede bone integration. It can also lead to loosening of the prosthesis, which in severe cases requires revision surgery.

Post-treatment and rehabilitation

Proper aftercare is crucial to ensure the optimal function of the cementless hip replacement and minimize the risk of disturbed ingrowth. Here are some of the essential aspects:

• Early mobilization: The patient should get up and put weight on the leg with the help of physiotherapists on the first day after surgery, albeit within a controlled framework. Too early or too heavy a load can disrupt the ingrowth.
• Controlled loading: Patients should only put as much weight on the operated leg as recommended by the surgeon in the first few weeks. As a rule, this is done gradually, starting with partial loading and gradual increase.
• Regular follow-up examinations: Regular X-ray checks are necessary to monitor the progress of ingrowth and detect possible complications early.

After a cementless total hip replacement (THR), post-operative care is crucial to support the successful ingrowth of the prosthesis and avoid long-term complications. In the first six weeks after surgery, the focus should not be on intensive exercises, but rather on promoting bone and wound healing. There are clear recommendations on which loads and exercises should be avoided to support the healing process.

1. Avoidance of single-leg stance exercises

In the first six weeks after surgery, exercises that require standing on one leg should be avoided. This type of exercise leads to uneven stress on the operated leg and can potentially destabilize the primary fixation of the prosthesis.

Reasons for this:

• High load: The single-leg stance exposes the operated leg to a significant load, as the entire body weight acts on this leg. In the early phase, however, a controlled partial load is crucial to avoid compromising the ingrowth of the bone into the prosthesis.
• Instability: The muscles around the hip joint need to recover after surgery. Exercises in single-leg stance put excessive strain on the not yet fully regenerated muscles and could lead to unwanted movements in the hip joint.

Instead, patients should focus on exercises that engage both legs simultaneously and only partially load the operated leg.

2. Avoidance of unstable surfaces and wobbly plates

Unstable training devices, such as wobble boards, which are often used in physiotherapy to promote balance and stability, should be strictly avoided in the first six weeks. These devices can be helpful in the long term, but in the initial phase after hip replacement surgery, there is a high risk that they will lead to uncontrolled movements and disrupt the integration of the prosthesis.

Risks with loose plates:

• Micro-movements of the prosthesis: Standing on a wobble board requires constant small corrective movements of the body, which in turn could cause micro-movements of the prosthesis in the bone. Such movements are particularly critical during the ingrowth phase as they can affect primary stability.
• Delayed osseointegration: Uncontrolled movements and repeated load changes can delay or even prevent osseointegration (bone ingrowth into the prosthesis), leading to loosening of the prosthesis in the worst case.

3. Avoidance of impact loads

Impact loads, such as those that occur during jumping, running, or intensive strength training, should also be avoided in the early postoperative phase. Impact loads exert considerable pressure on the freshly operated hip joint and can hinder the ingrowth of the prosthesis.

Examples of shock loads:

• Running or jogging: The repetitive impact of running generates shock waves that are transmitted to the hip joint. Especially in the first few weeks after surgery, such movements are harmful as they jeopardize primary stability.
• Jumping or hopping: Similar loads occur when jumping, which is why exercises such as light hopping or jumping should not be performed in this phase.

Instead, attention should be paid to gentle movements and walking exercises with a gradual increase in stress.

4. Maximum strength and stability exercises in the early phase

Maximum strength exercises, i.e. exercises aimed at stressing the muscles to their limit, are not useful in the first six weeks after hip surgery. At this stage, it's not about restoring strength as quickly as possible, but about supporting bone healing and wound healing.

Reasons against maximum strength exercises:

• Increased muscle tension: maximum strength exercises put the muscles around the hip under strong tension, which poses a risk that the prosthesis will be destabilized.
• Loading of the prosthesis anchorage: These exercises could excessively stress the fixation of the prosthesis in the bone and increase the risk of loosening.

Similarly, stability exercises are of minor importance, as the stability of the hip in the first weeks is primarily achieved through the bony ingrowth of the prosthesis and not through the strengthening of the surrounding musculature.

5. Respecting bone healing and wound healing

The primary goal of the first six weeks after a cementless hip replacement is to avoid disrupting the healing process of the bone and soft tissues. Bone healing plays a crucial role in the long-term ingrowth of the prosthesis, and therefore it is essential to avoid exercises and activities that could hinder this process.

Bone healing process:

• Primary healing phase: In the first weeks after surgery, the bone is in an acute healing phase, during which new bone substance is formed. Excessive or incorrect stress can interrupt or slow down this process.
• Wound healing: In addition to bone healing, the healing of soft tissue around the hip is crucial. Exercises or movements that put too much stress on the surgical wound could lead to wound healing disorders.

A regulated, moderate build-up of stress promotes healing and supports the long-term function of the prosthesis.

Recommended exercises in the first six weeks

While intensive exercises and shock loads should be avoided in the first few weeks, there are still some useful exercises that can be performed to gradually rebuild mobility and muscle strength without jeopardizing healing.

Recommended types of movement:

• Walking: Gentle walking with the support of walking aids is one of the safest and most effective exercises in the first weeks after surgery. It promotes blood circulation and supports healing without putting too much strain on the hip.
• Leg raises in supine position: This exercise can help gently strengthen the muscles without putting excessive strain on the hip joint.
• Movement in water: After consulting with the doctor, light training in water can also be useful, as the water supports the body and reduces the load on the hip joint.

Conclusion

The cementless hip replacement is a proven method to provide patients with hip joint diseases a long-term solution. The success of the prosthesis depends largely on the primary stability and subsequent secondary stability. Controlled loading and careful aftercare are crucial to prevent disturbed ingrowth. With the right technique and aftercare, patients can benefit from a long lifespan of the prosthesis and a high quality of life.

In the first six weeks after a cementless hip replacement, patients should be cautious with loads and exercises. Single-leg stance, wobble boards, shock loads, and intensive strength exercises should be avoided as they increase the risk of disturbed healing and loosening of the prosthesis. Instead, the focus should be on gentle movements that promote healing and do not compromise the prosthesis. It is crucial to respect the healing process of the bone and soft tissues to ensure long-term stability of the prosthesis.