Inventory math for orthodontic clinics: what to actually track

Ortho inventory is unusual. The item count is high (brackets, wires, bands, elastics, retainers, aligners, the specialist consumables). The unit value varies by 100x (a band costs ₹15, an aligner case costs ₹50,000). The consumption pattern is uneven, both seasonally and per-doctor. And running out of the wrong item at the wrong time costs you a chair-hour, which is roughly ₹3,000 to ₹7,500 of revenue.

Most clinics handle this by stocking heavily and reordering by feel. This works, kind of, until cash flow gets tight or the cupboard fills up. Then the owner asks "are we stocking too much" and discovers no one has a clear answer.

Here is the math that actually answers the question.

The three numbers per item

For every single inventory item, you need three numbers:

1. Average daily consumption. How many of this item does the clinic use per day, averaged over the last 90 days.
2. Lead time in days. Number of days between placing an order with your supplier and the stock arriving on your shelf.
3. Variability: how much daily consumption fluctuates. A simple proxy is the standard deviation of daily consumption. Most clinics can get away with "max consumption in a busy day" as a substitute.

With these three numbers, every other inventory question has a mechanical answer.

Reorder point

The reorder point is the stock level at which you place a new order. It is:

reorder point = (average daily consumption x lead time) + safety stock

Safety stock is the buffer. It is roughly:

safety stock = (max daily consumption - average daily consumption) x lead time

Worked example for a common item, 0.018 nickel-titanium archwires:

• Average daily consumption: 4 wires
• Max daily consumption (busiest day in last 90): 9 wires
• Lead time: 7 days

Reorder point = (4 x 7) + ((9 - 4) x 7) = 28 + 35 = 63 wires.

When stock hits 63, place the order. Not when it hits 30 because "we usually have enough". Not when it hits 100 because "the supplier is unreliable". 63 is the math. Run the math.

Order quantity

How much to order each time. The classical answer is the Economic Order Quantity, which optimises for a balance between ordering cost and holding cost. In practice, most ortho clinics simplify to:

order quantity = average daily consumption x reorder cycle

Where reorder cycle is how often you want to be reordering this item. Common choices:

• High-velocity items (used daily, easy to source): reorder weekly.
• Medium-velocity items: reorder every 2 to 4 weeks.
• Low-velocity items: reorder monthly or quarterly.

For the archwire example, with average daily consumption of 4 and a 14-day reorder cycle, order quantity = 56 wires per order.

Total stock holding

Once you have reorder point and order quantity, your stock at any given time fluctuates between:

• Just before delivery: safety stock + (a few days' average consumption that arrived since the order was placed)
• Just after delivery: reorder point + order quantity

For the archwire example: stock fluctuates between roughly 35 and 119 wires, average around 75.

Multiply this average by the unit cost across all items and you have your total inventory cash. This is the number that should be 25 to 35 percent lower than what most clinics actually hold.

Where most clinics overstock

Three common patterns:

1. Bulk buying for a discount. "If we order 200, the supplier gives 10% off." For most items this is bad math. The 10% saving is wiped out by the holding cost (cash tied up, expiry risk, storage space) within 6 months. Run the comparison: is the discount more or less than 12 months of holding cost? Often it is less.
2. Stockpiling against supply scares. A bad delivery experience six months ago is causing the clinic to carry double safety stock now. Memory is not data. Reset to actual numbers.
3. Doctor-specific consumables nobody clears. A doctor who has not been with the clinic for a year still has their preferred bracket type sitting in a drawer. Audit annually. Donate, return, or write off.

Where most clinics understock

Two patterns, both worse than overstocking:

1. Consumables that hurt when missing. Local anaesthetic, suture material, infection control supplies. Stock-out cost here is not just chair-time, it can be a cancelled procedure or worse. Carry generous safety stock for these. Do not optimise.
2. Long-lead-time aligner shipments. If your aligner shipments have a 3-week lead time and you only realise you are about to run out 5 days before a planned start, you have a problem. These items need their reorder points calculated based on the long lead time, not the short one for everyday consumables.

Tracking expiry, not just count

For a meaningful chunk of ortho inventory, expiry matters. Anaesthetic, infection control supplies, certain bonding materials. An item is functionally out of stock the day it expires, regardless of count.

The fix is to track inventory in batches with expiry dates, and to use first-expiring-first-out as the consumption rule. The system raises a flag at 90 days before expiry, then 30 days, then 7. Each flag is a chance to use the stock before it has to be written off.

Manual expiry tracking via spreadsheet is theoretically possible. In practice, every clinic that tries finds at least one expired batch in the cupboard within 6 months. The overhead is not worth it. A real inventory system handles batches automatically.

What about the high-value items: aligner cases?

Aligner cases (and full custom retainers) are the exception that proves the rule. They are not inventory in the conventional sense. They are made-to-order per patient. There is no reorder point, no safety stock.

What you track for aligner cases is:

• Order placed date (when you ordered from the lab).
• Expected delivery date (lab's commitment).
• Patient appointment date (when you need the case in hand).
• Buffer: expected delivery date should be at least 5 working days before the patient appointment.

The metric is "cases on hand at appointment date", not "cases in stock". Different problem, different math.

Putting it together

The inventory operating rhythm for an ortho clinic that has this dialled in:

• Daily: someone glances at the reorder flag list. Most days there is nothing. Some days there is one item.
• Weekly: place orders for everything that has crossed the reorder point. One supplier interaction per week, not one per item.
• Monthly: 15-minute review of consumption patterns. Anything that has shifted more than 20 percent from the 90-day average gets its reorder point recalculated.
• Quarterly: full audit. Everything in the cupboard is counted. Discrepancies between physical and system stock are investigated. Expired items are written off.
• Annually: full reset of reorder points and order quantities based on the last 12 months of consumption.

This rhythm takes about 4 hours of staff time per week, total. Most clinics spend more than that today, distributed across "is anyone going to order more elastics" conversations and emergency runs to the supplier.

The honest summary

You do not need a bigger spreadsheet. You need three numbers per item and a daily 5-minute glance at a flag list. The clinics that move from "feels right" to "math says this much" find that they have 25 to 35 percent more cash unlocked, fewer stock-outs, and one fewer recurring source of stress in the practice.