Biological fixation refers to two very different processes depending on the context. In medicine, it describes bone naturally growing into or onto a surgical implant to hold it in place without cement. In ecology and agriculture, it describes microorganisms converting nitrogen gas from the atmosphere into a form plants can use. Both meanings share a core idea: a living system does the anchoring or converting work, rather than a synthetic or industrial process.
Biological Fixation in Joint Replacement Surgery
When surgeons replace a hip, knee, or other joint, the artificial implant needs to stay firmly attached to your skeleton for decades. One approach uses bone cement to glue the implant in place. The alternative, biological fixation, skips the cement entirely and instead relies on your own bone to grow directly into or onto the implant surface, creating a natural bond between living tissue and metal.
This bone-to-implant connection is called osseointegration, a term first defined by the Swedish researcher Per-Ingvar Brånemark as “a direct structural and functional connection between ordered living bone and the surface of a load-carrying implant.” The key feature is that no fibrous scar tissue sits between the bone and the metal. Instead, healthy bone locks directly onto the implant’s surface, transferring weight and stress the same way bone normally does.
Two slightly different versions of this process exist. Bone ingrowth happens when bone fills the tiny holes in a porous-coated implant surface, physically interlocking with the metal. Bone ongrowth happens when bone attaches to a roughened but non-porous surface. Both achieve the same goal: a stable, long-lasting bond without cement.
How Implants Encourage Bone Growth
Your bone won’t grow onto just any metal surface. The implant needs a specific texture, or “topography,” that mimics the kind of rough, porous structure bone cells recognize and colonize. Manufacturers create this using titanium alloy surfaces with interconnected pores typically ranging from about 150 to 200 micrometers in diameter, roughly the width of two human hairs. That pore size matches what bone cells need to migrate inward and lay down new tissue.
Many implants also receive a coating of hydroxyapatite, a calcium phosphate ceramic that is chemically similar to the mineral component of natural bone. When applied to porous titanium, hydroxyapatite lines the inner surfaces of the pores and acts as a biological signal, encouraging bone cells to attach, multiply, and fill the space. This combination of physical structure and chemical coating gives cementless implants their ability to integrate with living bone.
The Timeline for Bone Ingrowth
Biological fixation doesn’t happen overnight. It unfolds in two distinct phases. During the first three months after surgery, bone ingrowth accelerates rapidly, with roughly 48% of the total ingrowth occurring in this window. By the three-month mark, the mechanical attachment is already strong enough to provide functional stability.
From three months to one year, a slower but steady phase continues, adding approximately another 23% of ingrowth. At 12 months, the attachment strength is roughly 2.5 times what it was at three months. This is why recovery protocols for cementless joint replacements typically involve a period of limited weight-bearing early on: the implant needs time to develop its biological anchor before it can handle full daily loads.
Who Benefits Most From Cementless Implants
Cementless biological fixation tends to be favored for younger, more active patients with good bone quality. The logic is straightforward: a younger person may outlive a cemented implant, and biological fixation has the potential to last longer because bone continuously remodels and maintains the bond over time. For patients under 55, data from the New Zealand Joint Registry shows uncemented hip replacements actually have a lower revision rate (0.89 per 100 component years) compared to cemented ones (1.73 per 100 component years).
For patients over 70, cemented fixation is often preferred because bone quality tends to decline with age, making reliable ingrowth less certain. Registry data shows cemented total hip replacements have a 20-year survival rate of about 85%. The choice between cemented and cementless ultimately depends on your age, activity level, and bone density.
Smoking is one of the strongest risk factors working against biological fixation. Smokers are 37% less likely to achieve full bone healing within two years compared to nonsmokers, and they face significantly higher rates of delayed union and complications. The number of cigarettes smoked per day directly correlates with the risk of needing reoperation. Former smokers carry some increased risk too, though less than current smokers.
Biological Fixation in Dental Implants
The same principle applies in dentistry. A titanium post is placed into the jawbone, and over several months, bone grows directly onto the implant surface to hold it permanently. The American Academy of Implant Dentistry defines successful osseointegration as “contact established without the interposition of nonbone tissue between normal remodeled bone and an implant, entailing a sustained transfer and distribution of load from the implant to and within the bone tissue.” In practical terms, this means the implant doesn’t move under chewing forces, and healthy bone and marrow are visible right up against the metal surface with no gap of soft tissue in between.
Biological Nitrogen Fixation
Outside of medicine, biological fixation most commonly refers to nitrogen fixation: the process by which certain microorganisms pull nitrogen gas (N₂) from the atmosphere and convert it into ammonia (NH₃), a form of nitrogen that plants can absorb and use to build proteins, DNA, and other essential molecules. This matters because nitrogen gas makes up 78% of the atmosphere, but plants cannot use it in that form. Without fixation, life on Earth would run out of usable nitrogen, since natural processes continuously lock existing nitrogen into ocean sediments or convert it back to atmospheric gas.
The enzyme responsible is nitrogenase, first named in 1934, and it exists only in a select group of bacteria and archaea. No plant or animal can fix nitrogen on its own. The industrial alternative is the Haber-Bosch process, which uses extreme heat and pressure to accomplish the same chemical conversion. About 100 million tons of nitrogen are fixed industrially each year to produce fertilizer for global food production.
Bacteria That Fix Nitrogen
Nitrogen-fixing bacteria fall into two broad categories based on how they live. Symbiotic fixers, most famously Rhizobium and related genera, form a partnership with legumes like soybeans, peas, clover, and lentils. The bacteria colonize the plant’s roots, forming small nodules where fixation takes place. The plant provides the bacteria with sugars and a low-oxygen environment (nitrogenase is destroyed by oxygen), and in return the bacteria supply the plant with usable nitrogen. This is why farmers rotate crops with legumes: the plants naturally replenish soil nitrogen.
Interestingly, one non-legume plant, Parasponia (a tropical tree related to hemp), can also form root nodules with Rhizobium. It remains the only known non-legume capable of this type of symbiosis, and recent studies show it can be nodulated by bacteria from at least four different genera.
Free-living (non-symbiotic) nitrogen fixers include Azotobacter, Azospirillum, and several other genera that live in soil without requiring a plant host. These bacteria associate with the root zones of grasses, cereals like wheat and maize, and other crops, contributing smaller but meaningful amounts of fixed nitrogen. Despite the diversity of nitrogen-fixing organisms, symbiotic fixation by legumes currently accounts for only about 13% of total fertilization on arable land worldwide, with industrial fertilizer making up the bulk.
Biological Carbon Fixation
A less commonly searched but related process is biological carbon fixation, where living organisms convert carbon dioxide from the atmosphere into organic compounds. The most widespread pathway is the Calvin cycle, used by plants, algae, and cyanobacteria during photosynthesis. It is the single most important carbon-fixing process on the planet and the foundation of nearly all food chains.
At least five alternative carbon fixation pathways exist in various microorganisms, most of which incorporate CO₂ into existing carbon-containing molecules using different enzymes than the Calvin cycle. A sixth ancient pathway, the reductive acetyl-CoA pathway, is found in microbes that live without oxygen. These alternative routes are of growing interest because they offer different efficiencies and could potentially be harnessed to capture atmospheric carbon dioxide at scale.