When doctors replace a knee, sometimes the thigh bone breaks near the new joint. Researchers looked at 3,817 knee replacement patients to understand why this happens and how to fix it best. They found that patients with low vitamin D and calcium levels, plus certain positioning of the artificial knee, were more likely to get these fractures. When fractures did occur, using two metal plates to fix them worked better than using just one plate. This research helps doctors prevent these complications and choose the best treatment when they happen.

The Quick Take

  • What they studied: Why some people get thigh bone fractures after knee replacement surgery, and whether using one or two metal plates to fix these fractures works better
  • Who participated: 3,817 patients who had knee replacement surgery at one hospital between 2009 and 2022. Of these, 39 patients developed fractures near their new knee joint, and 40 patients without fractures were compared to them
  • Key finding: Patients with lower vitamin D and calcium levels were more likely to get fractures. When fractures happened, using two metal plates to hold the bone together prevented re-breaking, while five patients with just one plate broke their bones again
  • What it means for you: If you’re having knee replacement surgery, maintaining good vitamin D and calcium levels may help prevent complications. If a fracture does occur, your doctor should consider using two plates instead of one for better healing

The Research Details

Researchers looked back at medical records from 3,817 patients who had knee replacement surgery over 14 years. They identified 39 patients who developed fractures near their artificial knee joint and carefully matched them with 40 patients who didn’t have fractures. The matching process considered age, weight, sex, and health conditions to make fair comparisons.

They measured several things in both groups: vitamin D and calcium blood levels, bone density, how the artificial knee was positioned, and the alignment of the leg. They also tracked which patients got one metal plate versus two plates to fix their fractures, and watched them for several years to see if the bones broke again.

This type of study is called a case-control study because it compares people who had a problem (fractures) with similar people who didn’t have the problem, looking backward through their medical history.

By comparing patients who got fractures with very similar patients who didn’t, researchers can identify what’s different about the fracture group. This helps doctors spot warning signs before surgery and know which patients need extra attention. The comparison of one-plate versus two-plate fixation is important because it shows which surgical approach actually prevents bones from breaking again.

This study is reliable because it included a large number of patients (3,817) from one hospital system, giving consistent medical records. The researchers used a matching process to make sure the fracture and non-fracture groups were as similar as possible, except for whether they got fractures. However, because it looks backward at medical records rather than following patients forward, it cannot prove that low vitamin D definitely causes fractures—only that they’re connected. The study was conducted at a single hospital, so results might differ in other locations.

What the Results Show

The researchers found that vitamin D and calcium levels were significantly lower in patients who developed fractures compared to those who didn’t. This suggests that bone health measured by these nutrients may play a role in fracture risk after knee replacement.

When looking at how the artificial knee was positioned, they found that the angle of the knee component (the artificial part) in the front-to-back direction was different between the two groups. Patients with lower angles in this direction were more likely to get fractures. The overall leg alignment also differed between groups in a way that affected fracture risk.

Interestingly, some things doctors thought might matter—like notching of the bone during surgery, the overall leg alignment angle, and the side-to-side positioning of the knee component—did NOT significantly differ between the fracture and non-fracture groups.

The most striking finding involved treatment: five patients treated with a single metal plate experienced re-fractures (bones breaking again) an average of 19 months after surgery. In contrast, zero patients treated with two metal plates had re-fractures during the follow-up period.

The study found that various health conditions (comorbidities) and body weight were similar between the two groups, suggesting these weren’t major factors in fracture development. Bone density measurements also didn’t significantly differ, though vitamin D and calcium levels did, indicating that blood nutrient levels may be more important than bone density measurements for predicting fracture risk in this situation.

This research adds to existing knowledge by identifying specific nutrient deficiencies (vitamin D and calcium) as risk factors for fractures after knee replacement. Previous studies have focused more on surgical technique and bone quality, but this work emphasizes the importance of nutritional status. The finding that two-plate fixation prevents re-fractures is consistent with biomechanical principles suggesting that more support helps bones heal better.

This study looked backward at medical records rather than following patients forward, so it cannot prove cause-and-effect relationships. The research was done at a single hospital, so results might differ elsewhere. The number of patients who got fractures (39) is relatively small, which limits how much we can generalize the findings. The study didn’t measure vitamin D and calcium levels in all patients before surgery, only in those who developed fractures, which could bias the results. Finally, the follow-up time varied between patients, so some may not have been watched long enough to detect late complications.

The Bottom Line

Before knee replacement surgery, ask your doctor to check your vitamin D and calcium levels and correct any deficiencies (moderate confidence). If you do develop a fracture near your knee replacement, ask your surgeon about using two metal plates rather than one for better healing (moderate-to-high confidence based on this study). Maintain adequate vitamin D and calcium intake after surgery through diet or supplements as recommended by your doctor (moderate confidence).

People planning to have knee replacement surgery should pay attention to this research, especially those with known vitamin D or calcium deficiencies. Patients who develop fractures near their knee replacement should definitely discuss two-plate fixation with their surgeon. Orthopedic surgeons should consider this information when planning surgery and managing complications. People with osteoporosis or other bone health concerns should be especially attentive to nutrient levels before knee surgery.

Fracture prevention benefits from adequate vitamin D and calcium would likely take weeks to months to develop before surgery. If a fracture occurs and is treated with two plates, bones typically begin healing within 6-12 weeks, with full healing taking 3-6 months. The study followed patients for an average of 19 months, so benefits of two-plate fixation in preventing re-fractures were seen within this timeframe.

Want to Apply This Research?

  • Track vitamin D and calcium intake daily (in milligrams) through food and supplements, with a goal of 1,000-1,200 mg calcium and 600-800 IU vitamin D daily for adults. Log blood test results when available to monitor actual levels.
  • Set reminders to take vitamin D and calcium supplements at the same time each day. Log dietary sources of these nutrients (dairy, leafy greens, fortified foods) to ensure adequate intake. Schedule regular blood tests to monitor levels, especially before planned surgery.
  • Create a long-term tracking dashboard showing monthly vitamin D and calcium intake versus recommended amounts. Set alerts for upcoming blood tests to monitor nutrient levels. If surgery is planned, track compliance with supplementation in the months leading up to the procedure. Post-surgery, continue monitoring intake and any symptoms of bone pain or instability.

This research describes patterns found in one hospital’s patient records and cannot prove that low vitamin D causes fractures or that two plates will definitely prevent re-fractures in all patients. Individual results vary based on many factors including age, overall health, bone quality, and surgical technique. Before knee replacement surgery, discuss your personal fracture risk with your orthopedic surgeon. If you develop a fracture near a knee replacement, work with your surgical team to determine the best treatment for your specific situation. Do not start, stop, or change vitamin D or calcium supplementation without consulting your doctor, as individual needs vary. This information is educational and should not replace professional medical advice.